Dozens of people were killed in rocket strikes by Russian forces on Kharkiv, Ukraine's second largest city, on Monday morning, a Ukrainian minister said.
Russia faces deepening economic isolation four days after invading Ukraine in the biggest assault on a European state since World War Two.
"Kharkiv has just been massively fired upon by grads (rockets). Dozens of dead and hundreds of wounded," Interior Ministry Advisor Anton Herashchenko said in a post on Facebook.
The city, located in eastern Ukraine, has imposed a curfew in the wake of the heavy bombardment.
Talks between Russian and Ukrainian officials began on the Belarusian border earlier, with the aim of an immediate ceasefire and the withdrawal of Russian forces, the Ukrainian president's office said, after the Russian advance has gone more slowly than some expected.
Neighboring Belarus could send troops to help Russia as soon as Monday, according to a senior American intelligence official with direct knowledge of current U.S. intelligence assessments. The official spoke on condition of anonymity because he was not authorized to speak publicly.
Russia's Defense Ministry, meanwhile, said its nuclear missile forces and Northern and Pacific fleets had been placed on enhanced combat duty - in line with an order the previous day from Russian President Vladimir Putin.
Putin on Sunday ordered his military command to put Russia's deterrence forces - which include nuclear arms - on high alert, citing what he called aggressive statements by NATO leaders and Western economic sanctions against Moscow.
At least 102 civilians in Ukraine have been killed since Thursday, with a further 304 wounded, but the real figure is feared to be "considerably higher", U.N. human rights chief Michelle Bachelet said on Monday.
More than half a million people have fled to neighboring countries, according to the UN Refugee Agency.