Talks between Russian and Ukrainian officials began on the Belarusian border on Monday, Moscow said, as Russia's diplomatic and economic isolation deepens four days after invading Ukraine, the biggest assault on a European state since World War Two.
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.
Talks began with the aim of an immediate ceasefire and the withdrawal of Russian forces, the Ukrainian president's office said, read more after a Russian advance that has gone more slowly than some expected.
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.
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 was sweeping, with sanctions that effectively cut off Moscow's major financial institutions from successive Western markets sending Russia's rouble currency down 30 per cent against the dollar on Monday. Countries also stepped up weapons supplies to Ukraine.
Blasts were heard before dawn on Monday in the capital of Kyiv and in the major eastern city of Kharkiv, Ukrainian authorities said. But Russian ground forces' attempts to capture major 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 or 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.
A senior US defence official said Russia had fired more than 350 missiles at Ukrainian targets since Thursday, some hitting civilian infrastructure.
"It appears that they are adopting a siege mentality, which any student of military tactics and strategy will tell you, when you adopt siege tactics, it increases the likelihood of collateral damage," the official said, speaking on condition of anonymity.
