Russia's Black Sea flagship sank on Thursday after an explosion and fire that Ukraine claimed was a successful missile strike, as the Kremlin accused Kyiv of targeting its citizens in sorties across the border.

The Moskva missile cruiser had been leading Russia's naval effort in the seven-week conflict, in which civilian killings have sparked accusations of genocide from the United States and others.

Russia's defence ministry said the blast on the vessel was the result of exploding ammunition and added that the resulting damage had caused it to "lose its balance" as it was being towed to port.

"Given the choppy seas, the vessel sank," the Russian state news agency TASS quoted the ministry as saying.

On the Ukrainian side, Odessa military spokesman Sergey Bratchuk said the ship had been hit by domestic Neptune cruise missiles.

In Washington, Pentagon spokesman John Kirby said he was unable to verify either version, but stressed that the sinking of the Moskva dealt a "big blow" to the Black Sea fleet.

The fleet has been blockading the besieged southern port city of Mariupol, where Russian officials say they are in full control.

Following its pullout from northern Ukraine earlier this month after failing to take the capital, Russia is refocusing on the east, with Kyiv warning of bloody new clashes to come in the Donbas region.

And with Russian setbacks in the war mounting, the CIA warned that President Vladimir Putin could resort to using a tactical or low-yield nuclear weapon.

"We're obviously very concerned. I know President Biden is deeply concerned about avoiding a third world war, about avoiding a threshold in which, you know, nuclear conflict becomes possible," CIA director William Burns said in a speech in Atlanta.

But the United States is yet to see "a lot of practical evidence" of actual deployments that would cause more worry, he added.

- 'No electricity, no water' -

Seizing Donbas, where Russian-backed separatists control the Donetsk and Lugansk areas, would allow Moscow to create a southern corridor to the occupied Crimean peninsula. 

But rain that has been battering the region for days could favour Ukraine in its fight against invading Russian forces, a senior Pentagon official said Thursday.

"The fact that the ground is softer will make it harder for them to do anything off of paved highways," said the official, who spoke under condition of anonymity.

In what appeared to be its first official accusation of abuses targeting Russians, the Kremlin said at least six air strikes had hit residential buildings in the border region of Bryansk, wounding seven people including a toddler.

"Using two military helicopters carrying heavy weaponry, Ukrainian armed forces illegally entered Russian air space," Russia's Investigative Committee said. 

Russia sparked fears of a return to conflict around Kyiv on Wednesday when it threatened to attack the capital's strike command centres in retaliation for any strikes on Russian soil.

In the south and east, civilian evacuations had been set to resume Thursday, Deputy Prime Minister Iryna Vereshchuk said, after a day-long pause that Kyiv blamed on Russian shelling.

Tamara Yakovenko, 61, and her 83-year-old mother had decided to run the risk of fleeing Severodonetsk, the last easterly city still held by Ukrainian forces, where "every 10 or 15 minutes there are bombings". 

"We used to receive humanitarian aid, but now nobody remembers us. Some people try to cook outside on a fire... And boom, boom... everyone has to run back to the basement," Yakovenko said.

"All night until morning, there is no rest."

Now little more than a ghost town just kilometres from the front line, Severodonetsk has already buried 400 civilians, according to Lugansk regional governor Sergiy Gaiday.  

"There's no electricity, no water," Maria, who lives with her husband and mother-in-law, told AFP amid a din of shelling that she said never stops.

"But I prefer to stay here, at home. If we leave, where will we go?"

- Global hunger -

Beyond the humanitarian crisis, the war's economic consequences -- primarily surging food and fuel prices -- were "hitting hardest the world's most vulnerable people," IMF Managing Director Kristalina Georgieva warned.

The United Nations announced the release of $100 million to fight hunger in Yemen and six African countries at risk of famine due to the war disrupting food supply chains.

"Hundreds of thousands of children are going to sleep hungry every night while their parents are worried sick about how to feed them," said UN Emergency Relief Coordinator Martin Griffiths.

"A war halfway around the world makes their prospects even worse. This allocation will save lives."

Investigators have descended on areas around Kyiv previously occupied by Russian forces, looking into reports of war crimes that President Putin has dismissed as "fakes".

The atrocities -- some of which were witnessed by AFP -- have led Biden to accuse Putin of genocide, a term key European partners including France and Germany have hesitated to use.

The French government, which has allocated 100 million euros for humanitarian support to victims of the conflict, said its embassy would return "very soon" to Kyiv from the western city of Lviv, where it had been relocated after the invasion.

Latest developments

Here are the latest developments in the war in Ukraine:

- Russia accuses Ukraine of attacks -

Russian officials accuse Ukraine of sending two helicopters across the border to bomb a town in Russia's southern Bryansk region, after initially reporting seven injured in shelling.

Moscow says at least six residential buildings were damaged and that a toddler was among the injured. 

It is not possible to immediately verify the report.

Elsewhere, the governor of Russia's Belgorod region claims that the village of Spodaryushino, "came under shelling" from the Ukrainian side of the border and that it and a nearby village had been evacuated as a precaution.

- Ukraine resumes evacuations -

Ukraine says it is reopening humanitarian corridors through nine routes in the east and south, to facilitate the evacuation of civilians from war-scarred regions after a day-long pause that Kyiv attributed to Russian violations.

- CIA nuclear warning -

Russia's setbacks could lead President Vladimir Putin to resort to using a tactical or low-yield nuclear weapon, CIA director William Burns says.

"Given the potential desperation of President Putin and the Russian leadership, given the setbacks that they've faced so far, militarily, none of us can take lightly the threat posed by a potential resort to tactical nuclear weapons or low-yield nuclear weapons," Burns says.

- Genocide debate -

Leaders on either side of the Atlantic diverge on whether to label Russia's actions in Ukraine as "genocide".

Canadian Prime Minister Justin Trudeau says US President Joe Biden, who has accused Russian President Vladimir Putin of genocide, was "right" in his choice of words.

But French President Emmanuel Macron, who is campaigning for re-election, said such "verbal escalations" were unhelpful, and German Chancellor Olaf Scholz steers clear of using the term. 

- $800m US aid package -

The United States unveils a major new package of aid to Ukraine, including equipment such as helicopters, howitzers and armoured personnel carriers.

The package includes equipment Washington had previously refused to provide to Kyiv for fear of escalating the conflict with nuclear-armed Russia.

- Kharkiv offensive continues -

Russia's offensive on Ukraine's second city of Kharkiv has claimed a further four lives, governor Oleg Synegubov says. The city near the Russian border has been on the eastern frontline since the start of the war and suffered massive destruction.

- Le Pen for NATO-Moscow rapprochement -

French far-right leader Marine Le Pen says she will back closer ties between NATO and Russia and pull Paris out of the alliance's military command if elected president in an April 24 runoff with Emmanuel Macron.

Following accusations she is too close to President Vladimir Putin, Le Pen said a "strategic rapprochement" is required and questions need to be asked about the role of the alliance after the end of the Warsaw Pact.

- US warns China -

US Treasury Secretary Janet Yellen warns that China's stance towards Russia and its invasion of Ukraine could affect countries' willingness to collaborate and trade with Beijing.

- Prisoner swap -

Ukraine says that 30 prisoners of war are being returned to the country as part of the most recent exchange of captives with Russia, following an order from President Volodymyr Zelensky.

It says in a statement on social media the swap involves five officers and 17 servicemen, along with eight civilians, including one woman.

- 4.7 million refugees -

More than 4.7 million Ukrainians have fled their country in the 50 days since Russia invaded, the United Nations says.