The mayor of the Ukrainian city of Mariupol said on Monday that the death toll from the Russian siege could exceed 20,000, or 5% of the population.
Mayor Vadym Boichenko also told NHK that up to 120,000 residents were forced to stay in the eastern port city.
He criticized Russian troops for preventing buses and vehicles from leaving Mariupol and ordering them to return to the city at their checkpoints.
A US think tank, the Institute for the Study of War, released its latest assessment on Sunday, saying Russian forces have isolated remaining Ukrainian forces in two main locations in Mariupol.
The Russian Defense Ministry announced on Monday that its forces carried out attacks in the Donetsk region and destroyed a Ukrainian military command facility with a Kinzhal hypersonic missile.
The leader of pro-Russian separatists in eastern Ukraine, Denis Pushilin, said on Monday they had fully captured the port of Mariupol.
Russian Foreign Minister Sergey Lavrov said on state television on Monday that the Russian military operation would stop only if the two sides reached a final agreement in the ceasefire negotiations.
A Ukrainian Defense Ministry spokesman said on Monday that Russia had completed preparations to launch major attacks on eastern Ukraine, citing intelligence from Western countries. The spokesman added that Ukrainian forces were ready to defend the region.
Meanwhile, the Russian presidential office said President Vladimir Putin was due to visit the Vostochny space center in the Russian Far East on Tuesday with his Belarusian counterpart Alexander Lukashenko. The office says the two leaders will hold talks.
The Belarusian government said the presidents plan to discuss the situation in Ukraine and the economic sanctions imposed on the two countries.