Fighting raged in the critical port city of Odessa and across Ukraine's east as fresh evacuations of civilians from war-ravaged Mariupol were expected Tuesday.

The United States was warning that Moscow was preparing to formally annex regions in the country's east, while the European Union told member states to brace for a complete breakdown in Russian gas supplies as it prepared a new package sanctions.

Russia's Foreign Minister Sergei Lavrov meanwhile sparked outrage by alleging Adolf Hitler may have "had Jewish blood", invoking a conspiracy theory in a bid to discredit Ukraine's President Volodymyr Zelensky -- who is of Jewish ancestry.

Israel -- which has sought to keep a delicate balance between the two sides since Russia's invasion of Ukraine -- condemned the remarks and summoned Moscow's ambassador. 

Zelensky also slammed Lavrov's remarks as "anti-Semitic", and said they showed Russia had "forgotten all the lessons of World War II".

"It is no coincidence that they are waging a so-called total war to destroy all living things, after which only the burned ruins of entire cities and villages remain," he added.

- Shift in strategy -

The war has seen Moscow, after failing to take the capital Kyiv, shift its two-month-old invasion to largely Russian-speaking areas and step up pressure on Odessa, a cultural hub that is a crucial port on the Black Sea.

Odessa's city council said that a Russian strike hit a residential building housing five people.

A 15-year-old boy was killed and a girl was hospitalised, the council said on Telegram.

"What did these children... threaten the Russian state with? And that's how they fight. That's all," Zelensky said in his video address.

Russia's invasion has killed thousands of people and displaced more than 13 million people in a war the scale of which has not been seen in Europe for generations.

Among the most battered cities is Mariupol, where an untold number have died and survivors have little access to food, water and medicine as Russia battles to connect the southern and eastern strips of land under its control.

Kyiv said more than 100 civilians were evacuated over the weekend from the sprawling Azovstal steel plant, the last holdout of Ukrainian forces in Mariupol, where soldiers and civilians have been sheltering in a maze of underground tunnels.

Sviatoslav Palamar, deputy commander of Ukraine's Azov unit, said that another 20 people were transferred out on Monday evening, but only after a five-hour delay as "the enemy's artillery caused new rubble and destruction."

And Mariupol's city council said evacuations would restart at 7:00 am local time (0400 GMT) on Tuesday.

Ukraine and Russia have been coordinating civilian evacuations with United Nations agencies and the International Committee of the Red Cross.

Ukrainian forces have recaptured some territory in recent days, including the village of Ruska Lozova, which evacuees said had been occupied for two months.

"It was two months of terrible fear. Nothing else, a terrible and relentless fear," Natalia, a 28-year-old evacuee from Ruska Lozova, told AFP after reaching Kharkiv, Ukraine's second biggest city.

But Kyiv has admitted that Russian forces have captured a string of villages in the east and has asked Western powers to deliver more heavy weapons to bolster its defences there.

Ukraine's defence ministry said Monday that its drones had sunk two Russian patrol boats near the Black Sea's Snake Island, which became a symbol of Ukrainian resistance after soldiers there rebuffed Russian demands to surrender.

- 'Sham referenda' -

The fresh onslaught came as the United States warned that Moscow was preparing imminently to annex both Lugansk and neighbouring Donetsk.

Pro-Russian separatists in the two regions declared independence in 2014, but Moscow has so far stopped short of formally incorporating them as it did that year with the Crimean peninsula.

"Russia plans to engineer referenda upon joining sometime in mid-May," said Michael Carpenter, the US ambassador to the Organisation for Security and Co-operation in Europe.

He said that Russia was considering a similar plan in a third region, Kherson, where Moscow has recently solidified control and imposed use of its ruble currency.

"We think the reports are highly credible," Carpenter told reporters in Washington.

As with Crimea, he vowed that the international community would not support Russian-dictated changes to Ukraine's borders.

"Such sham referenda -- fabricated votes -- will not be considered legitimate, nor will any attempts to annex additional Ukrainian territory," Carpenter said.

"But we have to act with a sense of urgency."

- Bracing for new sanctions -

Western powers have levelled unprecedented sanctions against Russia over the war while delivering money and weapons to Ukraine, including a $33 billion (31 billion euro) arms and support package announced by US President Joe Biden last week.

The European Commission will on Tuesday propose a new package of measures, including an embargo on Russian oil, officials said.

And British Prime Minister Boris Johnson will announce another £300 million ($376 million, 358 million euros) in military aid for Kyiv, his office said.

In a remote address to Ukraine's parliament -- the first by a foreign leader to the Verkhovna Rada since Russia invaded on February 24 -- Johnson is set to hail the country's resistance as its "finest hour".

After talks on Monday, the European Union warned member states to prepare for a possible complete breakdown in gas supplies from Russia, insisting it would not cede to Moscow's demand that imports be paid for in rubles.

Germany, Europe's largest economy, was heavily dependent on Russian gas prior to the war, but European views quickly hardened after the invasion.

EU and French officials said the 27-member bloc was united with Poland and Bulgaria, whose gas supplies were cut last week after they refused to pay in rubles.

Western nations have been trying to show support by reopening embassies in Kyiv that were closed due to the invasion, with Denmark the latest to make the move Monday.

Kristina Kvien, the US charge d'affaires, announced in the western city of Lviv that Washington hopes to have diplomats back in Kyiv by the end of May.

Latest developments

Here are the latest developments in the war in Ukraine:

- Israel slams Lavrov Hitler comments -

Israeli Foreign Minister Yair Lapid slams his Russian counterpart Sergei Lavrov for alleging Adolf Hitler may have "had Jewish blood" in a bid to discredit Kyiv. 

Lavrov's comments -- which invoked a conspiracy theory exploiting a gap in the dictator's ancestry -- see Israel summon Moscow's ambassador for "clarifications" and condemn the "unforgivable and outrageous statement".

- EU to propose new sanctions on Russia -

The European Commission is set to propose a new package of measures, including an embargo on Russian oil, officials say.

The anti-Russian measures will also target the country's largest bank, Sberbank, which will be excluded from the global banking communications system SWIFT, diplomats said.

- Boris Johnson to address Ukraine parliament -

UK Prime Minister Boris Johnson will deliver an address to Ukraine's parliament -- the first by a foreign leader since Russia invaded on February 24 -- where he is set to hail the country's resistance as its "finest hour".

He will also announce another 300 million pounds ($376 million, 358 million euros) in military aid for Kyiv, his office says.

- 'Russia plans to annex east' -

Russia is planning imminently to "annex" the two eastern regions of Ukraine battered by its invasion after failing to overthrow the government in Kyiv, a senior US official says.

Their plan is to annex the "Donetsk People's Republic" and "Lugansk People's Republic" to Russia using referenda on the question, says Michael Carpenter, US ambassador to the OSCE. 

- Teen killed in Odessa -

A 15-year-old boy is killed in a fresh Russian strike on Ukraine's Black Sea port of Odessa, the southern city's council says.

A missile hit a residential building that had five people inside, said the statement, adding that a girl had been hospitalised.

- EU readies for end to Russian gas supplies -

The European Union warns member states to prepare for a possible complete breakdown in gas supplies from Russia, insisting it will not cede to Moscow's demand that imports be paid for in rubles.

The European Commission will on Tuesday propose to member states a new package of sanctions to punish the Kremlin for its invasion of Ukraine, including an embargo on Russian oil, officials say.

- Top Russian general visits Ukraine -

Russia's top general, Valery Gerasimov, visited the Donbas front in the Ukraine war last week, a Pentagon official says, but reports that he was injured in a Ukrainian attack could not be confirmed.

- Russian clubs banned from Champions League -

Russian clubs have been banned by UEFA from participating in the Champions League and all other European competitions next season, European football's governing body announces.

- Bolshoi scraps some performances -

Russia's Bolshoi Theatre announces it is cancelling the performances directed by Kirill Serebrennikov and Timofey Kulyabin, who have both spoken out against Moscow's military campaign in Ukraine.

- Finnish group scraps nuclear plant deal -

Finnish-led consortium Fennovoima says it has terminated a contract with Russian group Rosatom to build Finland's third nuclear power plant, citing the Ukraine conflict.

"The war in Ukraine has worsened the risks for the project," Fennovoima says in a statement, also citing "significant delays".