(ANSA-AFP) - KRAMATORSK, APR 12 - Russian troops were aiming to take control of the city of Mariupol on Tuesday, part of an anticipated massive onslaught across eastern Ukraine, as defending forces tried desperately to hold them back. Russia is believed to be trying to connect occupied Crimea with Moscow-backed separatist territories Donetsk and Lugansk in Donbas, and has laid siege to the strategically located city, once home to more than 400,000 people. Ukrainian forces were "surrounded and blocked", tweeted Myhaylo Podolyak, an official from President Volodymyr Zelensky's office. But on Monday the Ukrainian army insisted that "the defence of Mariupol continues". "The connection with the units of the defence forces that heroically hold the city is stable and maintained," the Land Forces of Ukraine wrote on Telegram. In his nightly address, Zelensky made another plea to his allies for more weapons to boost the defence of the city. "We are not getting as much as we need to end this war sooner. To completely destroy the enemy on our land... in particular, to unblock Mariupol," he said. He made a similar appeal for military assistance to South Korea's National Assembly earlier in the day, telling lawmakers Russia had "completely destroyed Mariupol and burned it to ashes". "At least tens of thousands of Mariupol citizens must have been killed," he added. - Chemical weapons allegations - Late Monday, Britain said it was trying to verify reports that Russia had also used chemical weapons in the city. Western officials have previously expressed concerns that as the conflict drags into its seventh week, Russia could resort to such extreme measures. Ukrainian lawmaker Ivanna Klympush said Russia had used an "unknown substance" and that people were suffering from respiratory failure. But on messaging app Telegram, Petro Andryushchenko, an aide to the city's mayor, wrote that a chemical attack was not confirmed and that they were "waiting for official information from the military". Elsewhere in the east, heavy bombardment continued as civilians were urged to flee ahead of an expected Russian troop surge in the region. Eight people were killed by shelling in the northeastern city of Kharkiv, the head of the regional state administration Oleg Synegubov said. "Our Armed Forces firmly hold the defensive positions of Kharkiv and the region," he wrote on his Telegram channel. "That is why the Russian enemy continues to 'fight' with the civilian population due to its powerlessness." Russian forces are reinforcing around the Donbas region, notably near the town of Izyum, but have not yet launched a full offensive, Pentagon officials said Monday. They reported a Russian convoy had been observed heading for Izyum, an hour's drive north of Kramatorsk, saying it appeared to be a mix of personnel-carriers, armored vehicles and possible artillery. Ukraine's defence ministry said it believed a major assault would happen soon. (ANSA-AFP).