Russia's army has "fully occupied" the key Ukrainian city of Severodonetsk after weeks of fighting, its mayor said, as Russian President Vladimir Putin pledged to send nuclear-capable missiles to Belarus within months.

As the war enters its fifth month, the capture of Severodonetsk marks an important strategic win for Moscow, which is seeking to gain full control over the east of the country after failing in its early objectives.

The industrial hub was the scene of weeks of running battles before the Ukrainian army began withdrawing its outgunned forces to better defend the neighbouring city of Lysychansk.

"The city has been fully occupied by the Russians," Mayor Oleksandr Striuk said Saturday.

A few hours earlier, pro-Moscow separatists said Russian troops and their allies had entered Lysychansk, which faces Severodonetsk on high ground across the Donets river. Its capture would give Russia control of the entire Lugansk region of the Donbas.

"Street fighting is currently taking place," a representative of the separatists, Andrei Marochko, said on Telegram, in a claim that could not be independently verified.

Far from the embattled Donbas, meanwhile, a flurry of Russian missiles was striking targets in northern and western Ukraine.

"More than 50 missiles of various types were fired: air, sea and ground-based," Ukraine's air force command said Saturday, noting the difficulty of intercepting Russian models such as the Iskander. 

Ukrainian President Volodymyr Zelensky, who will address G7 leaders on Monday, said cities as far away as Lviv, near the Polish border, had been struck.

"This confirms ... that Ukraine needs more assistance with weapons, and that air defence systems -- the modern systems which our partners have -- should not be on the sites or in storage, but in Ukraine," he said in his daily address.

- Pull in Belarus - 

In Saint Petersburg on Saturday, Putin said Russia would deliver Iskander-M missiles capable of carrying nuclear warheads to Belarus in the coming months, as he received Belarusian leader Alexander Lukashenko.

He also offered to upgrade Belarus' warplanes to make them capable of carrying nuclear weapons, in comments broadcast on Russian television.

Putin has referred to nuclear weapons several times since his country invaded Ukraine on February 24, in what the West has seen as a warning to the West not to intervene.

Ukraine said it had come under "massive bombardment" Saturday morning from neighbouring Belarus which, although a Russian ally, is not officially involved in the conflict.

Twenty rockets "fired from the territory of Belarus and from the air" targeted the village of Desna in the northern Chernigiv region, Ukraine's northern military command said.

It said infrastructure was hit, but no casualties had yet been reported.

Belarus has provided logistic support to Moscow since its February 24 invasion, particularly in the first few weeks, and like Russia has been targeted by Western sanctions.

"Today's strike is directly linked to Kremlin efforts to pull Belarus as a co-belligerent into the war in Ukraine," the Ukrainian intelligence service said.

- 'Ukraine can win' -

Russia's weekend breakthrough in Severodonetsk came on the eve of a week of feverish Western diplomacy, as US President Joe Biden flew to Europe for a G7 summit that starts Sunday, and NATO talks later in the week.

"Ukraine can win and it will win, but they need our backing to do so," said British Prime Minister Boris Johnson in a statement on the eve of the summit. 

"Now is not the time to give up on Ukraine."

The Western allies will take stock of the effectiveness of sanctions imposed so far against Moscow, consider possible new aid for Ukraine, and begin turning their eye to longer-term reconstruction plans. 

The European Union offered a strong show of support on Thursday when it granted Ukraine candidate status, although the path to membership is long.

- Evacuating the Azot plant -

As in the southern port of Mariupol before it, the battle for Severodonetsk has devastated the city.

On Saturday, Mayor Striuk said civilians had begun to evacuate the Azot chemical plant, where several hundred people had been hiding from shelling.

"These people have spent almost three months of their lives in basements, shelters," he said. "That's tough emotionally and physically."

They would now need medical and psychological support, he added.

Pro-Moscow separatists said Russian forces and their allies had taken control of the Azot factory and "evacuated" more than 800 civilians sheltering there.

Since 2014, the mainly Russian-speaking Donbas has been partially under the control of pro-Moscow separatists, who set up self-declared breakaway republics in Lugansk and Donetsk.

- 'Finishing what they started' -

While millions of Ukrainians have fled their homes and their country since the invasion, most to neighbouring Poland, some foreigners have gone the other way to fight.

Russia said Saturday its troops had killed up to 80 Polish fighters in strikes on a factory in Konstantinovka in the Donetsk region, a claim that could not be verified.

Russia has also intensified its offensive in the northern city of Kharkiv in recent days. 

An AFP team on Saturday saw a 10-storey administrative building in the city centre hit by missiles overnight, causing a fire but no casualties.

It had already been bombed, prompting one soldier on the scene to note: "The Russians are finishing what they started."

Moscow tightens economic grip on southern Ukraine

Little appears to have changed for Alexei Andrusenko, the head of a foundry in Ukraine's southern city of Berdyansk, who is happy to have kept all his staff since Moscow took control of the city.

Andrusenko and his 50 or so employees continue showing up to work every morning to the grey building in the outskirts of the port city on the shores of the Sea of Azov.

But now the factory's produce -- once sold to Ukrainian or international steel groups -- will likely be bound for Russia and Kremlin ally Belarus. 

Since Russia sent troops into Ukraine on February 24 and captured territories in the south of the pro-Western country, Moscow has sought to strengthen their economic ties.

"We have no other supply chain," Andrusenko told AFP during a press trip organised by the Russian army.

He also raised concerns about the depleting stocks of their raw materials that previously came from neighbouring Mariupol, another key Ukrainian city on the shores of the Sea of Azov.

Andrusenko says they are "interested" in working with the Alchevsk steelworks, a large factory with over 10,000 employees that since 2014 has been under the control of pro-Russian separatists of eastern Ukraine's Lugansk region.

Before Russia sent troops to Ukraine, these deals would never have been possible. 

"The most important thing is to build the right supply chain and to be able to work," Andrusenko said.

- Port '100 percent ready' - 

The southern Ukrainian regions of Kherson and Zaporizhzhia have been largely under Russia's control since the first weeks of Moscow's military campaign, and are now being forcefully integrated into Russia's economy. 

The main economic asset of Berdyansk is its port, which has remained mostly intact unlike that of Mariupol, the scene of a devastating siege.

In late March, an attack attributed to Ukrainian forces reportedly sank a Russian warship in Berdyansk waters, but today the port is "almost 100 percent ready" to ship grain, says Alexander Saulenko, the Moscow-installed head of Berdyansk.

Ukraine has accused Russia and its allies of stealing its wheat, contributing to a global food shortage caused by grain exports blocked in Ukrainian ports.

According to Saulenko, grain will soon be shipped out from the port, since silos will need to be freed up for the new harvest.

"We have prospects for contracts with Turkey. Russia is an agricultural country, it has enough grain of its own so it would be more profitable to trade elsewhere," Saulenko said. 

But the most tangible influence of Moscow on the local economy is the introduction of Russia's national currency since last month.

"Now you can buy everything in both rubles and hryvna," Ukraine's currency, the pro-Russian official added. 

According to him, Berdyansk received some 90 million rubles ($1.7 million) from Russia, but state employees are still paid in hryvna and it is impossible to withdraw cash rubles from ATMs. 

- Ties with Russia 'resuming' -

Neighbouring Melitopol, about 100 kilometres (60 miles) west of Berdyansk that came under Russian control on March 1, also uses the Russian ruble that is delivered from Crimea, the Black Sea peninsula Moscow annexed from Ukraine in 2014.

"It's a two-currency zone.... The ruble is delivered thanks to the open road to Crimea. Commercial ties with Russia, interrupted after 2014, are resuming," says Melitopol's pro-Russian mayor, Galina Danilchenko. 

"People are happy to accept the ruble... I don't see any problems," she added, but for reporters on the press trip it was difficult to speak freely with the city's residents. 

Back at the Berdyansk foundry, 41-year-old worker Sergey Grigoryev says he just hopes to get paid his salary. 

"In cash, not to my card, because you can't withdraw from it. In hryvnas or in rubles -- I don't care".