Ukraine worked on Wednesday to secure territory reclaimed from occupying Russian forces in a swift counter-offensive and reiterated plans to win back all occupied regions, as US President Joe Biden predicted "a long haul" ahead.

Ukraine's Special Envoy for the Middle East and Africa Ambassador Maxim Sobh has praised Qatar's support for the sovereignty and territorial integrity of his country and its humanitarian aid to ease the impact of the current war in Ukraine.

Two officers guarding a polio vaccination team in northwest Pakistan were shot dead by unidentified assailants, police said Tuesday, the latest deaths in an ongoing campaign to eradicate the disease.

Russia and Ukraine signed a landmark deal on Friday to reopen Ukrainian Black Sea ports for grain exports, raising hopes that an international food crisis aggravated by the Russian invasion can be eased.

Ukraine said on Tuesday its forces were still holding out inside Sievierodonetsk and trying to evacuate civilians, after Russia destroyed the last bridge to the devastated eastern city in a potential turning point in one of the war's bloodiest battles.

Ukrainian defenders were fighting fiercely for "every metre" of Sievierodonetsk, President Volodymyr Zelenskiy said, as Russian forces destroyed a bridge to another city across the river, leaving stranded civilians with just one way out.

Bitter fighting raged in Sievierodonetsk, but the region's governor said Ukraine remained in control of an industrial area and chemical plant in the eastern city where hundreds of civilians are sheltering from incessant Russian shelling

Ukrainian troops were engaged in fierce street fighting with Russian soldiers in the industrial city of Sievierodonetsk, while other towns were under constant shelling, as the Kremlin pushed for control of the eastern Donbas region.

President Vladimir Putin warned the West that Russia would strike new targets if the United States started supplying Ukraine with longer-range missiles, the TASS news agency reported on Sunday.

Kyiv was rocked by several explosions early on Sunday, the mayor of the Ukrainian capital said, a day after officials said its troops had recaptured a swath of the eastern battlefield city of Sievierodonetsk in a counter-offensive against Russia.

Ukraine marked 100 days since Russia's invasion on Friday with fighting raging across the east of the country, where Moscow's forces are tightening their grip on the Donbas region.

Ukrainian forces were holding out in Sievierodonetsk on Tuesday, resisting Russia's all-out assault to capture a bombed-out wasteland that Moscow has made the principal objective of its invasion in recent days.