Kyiv - 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 night. 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. 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.