The leader of local pro-Russian forces says one third of the city of Severodonetsk in eastern Ukraine is already under their control.

The city is seen as Ukrainian forces' last stronghold in the region of Luhansk.

The leader of the pro-Russian militant group in the region, Leonid Pasechnik, made the remark in an interview with Russian state-run Tass news agency on Tuesday.

Britain's defense ministry said on Tuesday that street fighting is likely taking place on the outskirts of the city. It also indicated that Russia's next targets would include Kramatorsk, a key city in the eastern region of Donetsk.

On Monday, Russian President Vladimir Putin spoke with Turkish President Recep Tayyip Erdogan by telephone.

The Kremlin said Putin told Erdogan that Russia is ready to facilitate the unhindered sea transit of grain and other goods in coordination with Turkey. Putin said this applies to ports in the Black and Azov seas.

Putin reportedly said that if sanctions were lifted, Russia could export significant volumes of fertilizer and agricultural products.

Ukrainian President Volodymyr Zelenskyy said on Monday that his country cannot export 22 million tons of grain because Russia has blocked ports in the Black Sea and seized part of the coast of the Sea of Azov. He said the blockade is destabilizing the situation on a global scale, and food is becoming increasingly expensive in different countries.

Zelenskyy added that there is consequently a threat of famine in Africa, Asia and some European countries.

Leaders of the European Union countries will discuss the matter and food security at a summit in Belgium on Tuesday.