Street fighting raged Tuesday for control of Ukraine's flashpoint city of Severodonetsk, with the situation changing "every hour", an official said, as Kyiv warned its troops were outnumbered by Russian forces.

Just days ago, Moscow seemed close to taking the strategic industrial hub in the east but Ukrainian forces have managed to hold out.

"Our heroes are holding their positions in Severodonetsk. Fierce street fights continue in the city," Ukrainian President Volodymyr Zelensky said in a video address late Monday.

Concerns about a global food crisis also grew as Zelensky warned of tightening grain supplies -- Ukraine is a top producer of the commodity -- due to what Washington described as a Russian strategy of "blackmail".

Thousands of civilians have been killed and millions forced to flee their homes since President Vladimir Putin ordered Russian troops into Ukraine on February 24.

After being repelled from other parts of the country, including Kyiv, Russia has concentrated its assault on the eastern Donbas region and had been making slow but steady progress.

Severodonetsk -- the largest city still in Ukrainian hands in the Lugansk region of the Donbas -- has been the focal point in recent weeks, and officials described a fast-changing situation on the ground.

"The situation is changing every hour, but at the same time there's enough forces and resources to repel attacks," said mayor Oleksandr Striuk on Monday.

"We have hope, we have faith in our armed forces, no one's going to abandon" Severodonetsk, he added.

'More and stronger'

Zelensky warned Ukrainian forces in the key city were outnumbered and the Russians "are stronger," speaking to journalists after visiting frontline positions in Lysychansk, across the river from Severodonetsk.

Days earlier, officials said the Russians controlled more than 70 percent of the city but then Moscow's forces were pushed back, and see-saw battles have been raging since.

The US-based Institute for the Study of War said the Ukrainians' successful counterattacks highlighted "the declining combat power of Russian forces in Ukraine".

The Russian invasion of Ukraine, combined with supply chain snarls and climate change, has triggered stark warnings of global food shortages.

Moscow has blockaded the key black sea port of Odessa, and Zelensky said Ukraine had up to 25 million tonnes of grain that could not be exported.

"In the autumn that could be 70 to 75 million tonnes," said the president, whose country was the world's fourth biggest grain exporter before the war.

In Washington, US Secretary of State Antony Blinken added his voice to the criticism.

"Right now a Russian naval blockade in the Black Sea is preventing Ukraine's crops from being shipped to their normal destinations," he said.

Blinken said it was a deliberate strategy by Putin to force the rest of the world "to give in to him," and eliminate sanctions on Russia.

"In other words, quite simply put, it's blackmail," Blinken said

Speaking at a State Department conference on food security issues arising from the invasion of Ukraine, he also described as "credible" reports that Russia has stolen grain from Ukraine for resale.

Britain followed the United States by announcing Monday it would supply longer-range, mobile missile launchers to Ukraine's forces, which could improve Kyiv's fight against Russian firepower.

The British defense ministry said it would be supplying track-mounted M270 multiple rocket artillery units, which can strike targets up to 80 kilometers (50 miles) away with precision-guided rockets, double the reach of standard battlefield artillery.

The announcement came after Putin had warned that Moscow would hit new targets in Ukraine if the West supplied Kyiv with such weapons -- but did not specify which targets.

Sexual violence concerns

Russian forces pressed their offensive on several other fronts in the east of Ukraine, with Kyiv saying it had repulsed seven attacks around Donetsk and Lugansk.

The Russian defense ministry said its aircraft had hit three arms depots and a fuel storage facility near the village of Kodema, in the Donetsk region.

On Monday, Russian Foreign Minister Sergei Lavrov hit out at European countries that prevented his plane passing through their airspace, forcing him to cancel a visit to ally Belgrade.

Serbian daily Vecernje Novosti reported that NATO-members Bulgaria, North Macedonia and Montenegro had refused access to their airspace.

At a meeting of the UN Security Council, the United States and Europe urged Russia to stop alleged sexual violence by its army and proxies in Ukraine, allegations that Moscow denounced as "lies."

And in Washington, officials accused Russia of trying to "intimidate" American correspondents in Moscow, who were summoned by the Russian foreign ministry and threatened with reprisals because of US sanctions.