Russian and Ukrainian officials on Monday held talks on the possibility of a cease-fire as Russia's attack on its neighboring country entered the fifth day, with Kyiv reportedly demanding the withdrawal of Russian troops from all Ukrainian territory.

The talks, held near the Belarusian border, came after Russian President Vladimir Putin placed the country's nuclear forces on high alert in the face of continuing resistance from Ukraine's troops and the rollout of economic sanctions from the United States and its allies.

According to local media reports and other sources, the demand from Ukraine's representatives for Russian troops to pull out also referred to Crimea, which Russia annexed from Ukraine in 2014. Russia, for its part, sought recognition of its sovereignty over Crimea and the "demilitarization" of Ukraine.

Delegations of Ukraine and Russia arrive to attend peace talks between the two countries in the Gomel region of Belarus on Feb. 28, 2022. (Photo courtesy of the Foreign Ministry of Belarus/Anadolu Agency/Getty/Kyodo)

Russia began its invasion of Ukraine on Thursday, after having asserted that its own security has been under threat with the North Atlantic Treaty Organization's eastward expansion and the possibility of Ukraine's membership in the alliance.

Moscow's "demilitarization" request apparently indicates that it wants Ukraine to give up on joining NATO.

The delegations may meet again on the Belarusian-Polish border in a couple of days, Russian news agency Interfax reported, citing the Belarusian foreign minister.

Cease-fire negotiations came after Ukrainian President Volodymyr Zelenskyy agreed Sunday to send a delegation to meet Russian officials near the Ukraine-Belarus border.

The Ukraine government reportedly said Monday that dozens of people were killed by a Russian attack on Kharkiv, Ukraine's second-largest city, in the country's northeast.

The U.N. human rights chief said more than 100 civilians have been killed and hundreds wounded in five days of fighting -- warning that the figure was likely a vast undercount -- and Ukraine's president said at least 16 children were among the dead, according to The Associated Press.

More than 500,000 people have fled from Ukraine, another U.N. official said.

An official at the U.S. Defense Department assessed that about two-thirds of the Russian troops that had been amassed along Ukraine's borders are invading the country.

Russia's Defense Ministry on Sunday admitted for the first time that it had suffered casualties among its military ranks but said the number was much lower than the death toll on the Ukrainian forces' side.

It also said Russian forces have destroyed more than 1,000 military facilities in Ukraine.