The talks are aimed at an immediate ceasefire and the withdrawal of Russian forces, according to the Ukrainian president's office.

Russian forces seized two small cities in southeastern Ukraine and the area around a nuclear power plant, the Interfax news agency said on Monday, but ran into stiff resistance elsewhere.

The Western-led response to Russia’s military operation was sweeping, with sanctions that effectively cut off Moscow's major financial institutions from successive Western markets. Countries also stepped up weapons supplies to Ukraine.