Talks between Russian and Ukrainian officials have began on the Belarusian border as diplomatic isolation and economic sanctions on Russia deepen four days after invading Ukraine.
Talks began on Monday with Ukraine calling for an immediate ceasefire and the withdrawal of Russian forces.
Russia has been cagier, with the Kremlin declining to comment on Moscow’s aim in negotiations.
It was not clear whether any progress could be achieved after President Vladimir Putin on Thursday launched the assault and put Russia’s nuclear deterrent on high alert on Sunday.
Russian forces seized two small cities in southeastern Ukraine and the area around a nuclear power plant, the Interfax news agency said on Monday.
The talks are being held on the border with strong Russian ally Belarus, where a referendum on Sunday approved a new constitution ditching the country’s non-nuclear status at a time when the former Soviet republic has become a launch pad for Russian troops invading Ukraine.
The Western-led response to the invasion has been sweeping, with sanctions that effectively cut off Moscow’s major financial institutions from successive Western markets, sending Russia’s rouble currency down 30 percent against the dollar on Monday.
Countries also stepped up weapons supplies to Ukraine.
On Monday, blasts were heard before dawn in the capital Kyiv and in the major eastern city of Kharkiv, Ukrainian authorities said. But Russian ground forces’ attempts to capture key urban centres had been repelled, they added.
Russia’s defence ministry, however, said its forces had taken over the towns of Berdyansk and Enerhodar in Ukraine’s southeastern Zaporizhzhya region as well as the area around the Zaporizhzhya nuclear power plant, Interfax reported.
The plant’s operations continued normally, it said.
Ukraine denied that the nuclear plant had fallen into Russian hands, according to the news agency.
There was fighting around the Ukrainian port city of Mariupol throughout the night, Pavlo Kyrylenko, head of the Donetsk regional administration, said on television on Monday. He did not say whether Russian forces had gained or lost any ground, nor provide any casualty figures.
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”, UN human rights chief Michelle Bachelet said on Monday.
More than half a million people have fled to neighbouring countries, according to the UN refugee agency.
Reporting from Moscow, Al Jazeera’s Bernard Smith said Ukraine and Russia “seem very, very far apart on what they want out of the talks”.
“It seems Russia is continuing fighting, even while these talks are taking place … so both sides are very far apart and it it is hard to see how they can come to an agreement on a ceasefire.”