MOSCOW, April 21 (Reuters) - A top ally of Russian President Vladimir Putin said Russian forces will seize the last main stronghold of resistance in the besieged city of Mariupol on Thursday (Apr 21) after Ukraine proposed talks on evacuating troops and civilians there.

Mariupol would be the biggest city to be seized by Russia since invading Ukraine eight weeks ago in an attack that has taken longer than some military analysts expected, seen over five million people flee abroad and turned cities to rubble.

"Before lunchtime, or after lunch, Azovstal will be completely under the control of the forces of the Russian Federation," Ramzan Kadyrov, the head of Russia's republic of Chechnya, whose forces have been fighting in Ukraine, said of the steel plant.

Ukraine's defence ministry was not immediately available for comment.

A few dozen civilians managed to leave the strategically important southeastern port on Wednesday in a small bus convoy, according to Reuters witnesses, escaping the fiercest battle of the war.

A Ukrainian marine commander, Serhiy Volny, said fighters at the steel works may not be able to hold out for much longer. President Volodymyr Zelenskyy has said an estimated 1,000 civilians are sheltering there.

Ukraine is ready for a "special round of negotiations" with no conditions "to save our guys, (the far right) Azov (battalion), military, civilians, children, the living and the wounded," negotiator Mykhailo Podolyak tweeted.

Kyiv has proposed swapping Russian prisoners of war for safe passage for trapped civilians and soldiers. It was not known if Russia had responded to the offer of special negotiations.

Fighters remain holed up in the plant and have ignored an ultimatum by Russia to surrender. Fewer civilians than hoped left on Wednesday.

Ukraine said it had so far held off an assault by thousands of Russian troops attempting to advance in what Kyiv calls the Battle of the Donbas, a new campaign to seize two eastern provinces Moscow claims on behalf of separatists.

Russia's forces had carried out strikes on dozens of military facilities in eastern Ukraine and had shot down a Ukrainian Mi-8 helicopter near the village of Koroviy Yar, its defence ministry said.

Putin said a first test launch on Wednesday of Russia's Sarmat intercontinental ballistic missile, a new and long-awaited addition to its nuclear arsenal, would "provide food for thought for those who, in the heat of frenzied aggressive rhetoric, try to threaten our country."

Russia calls its incursion a "special military operation" to demilitarise and "denazify" Ukraine. Kyiv and its Western allies reject that as a false pretext for a war of choice.

G7 finance ministers at a meeting on Wednesday said Russia should no longer participate in international forums, including G20, International Monetary Fund and World Bank meetings this week.

Top officials from Britain, the United States and Canada walked out on Russia's representatives at a Group of 20 meeting in Washington, exposing deep divisions in the bloc of major economies.

The West has imposed unprecedented sanctions on Russia and Washington went further on Wednesday, slapping restrictions on dozens of people and entities including a commercial bank.