Ukraine's president agrees to talks as Putin raises the alert level on his nuclear forces

While the Ukrainian capital, Kyiv, was subjected to another night of heavy shelling, Russian forces entered Ukraine's second-largest city, Kharkiv.

On Sunday, Russian President Vladimir Putin ordered Russia's nuclear deterrent forces to be on high alert, raising tensions to a new high as Ukraine agreed to talks with Russian officials in neighboring Belarus after initially rejecting the idea.

Ukrainian President Volodymyr Zelenskyy said in a statement that he spoke with Belarus President Alexander Lukashenko and agreed to meet with Russians "without preconditions." Zelenskyy previously stated that his government would not attend talks in Belarus, a close ally of Moscow's where thousands of Russian troops gathered in the run-up to the invasion last week.

The delegations will meet near the Pripyat River on the Ukrainian-Belarusian border, according to Zelenskyy. It was unclear whether he would attend the talks or stay in Kyiv.

Russia, like NATO and the US, has thousands of nuclear warheads in its arsenal, and by raising the alert level, Putin explicitly raised the global stakes to a far deadlier level for the first time in this conflict.

The announcements by Zelenskyy and Putin came after Ukrainian and Russian forces engaged in pitched street battles in Kharkiv, Ukraine's second-largest city, and President Volodymyr Zelenskyy agreed to meet with a Russian delegation that had arrived in Belarus earlier that day.

As the fighting entered its fourth day, Kyiv residents awoke to find that the capital was still in Ukrainian hands.

"Anyone who wants to join the defense of Ukraine, Europe, and the world is welcome to come and fight alongside the Ukrainians," Zelenskyy's office said in a statement. "There is no greater sacrifice you can make for the sake of peace."

Dmytro Kuleba, Ukraine's foreign minister, echoed the call, inviting foreigners to contact Ukraine's foreign diplomatic missions in their respective countries.

According to regional officials, Russian vehicles broke into Kharkiv, a 1.4 million-person city 12 miles south of the Russian border.

According to officials, Russian forces blew up a gas pipeline in the city overnight. After the explosion sent a massive fireball into the sky, officials attempted to reassure residents that it was not a nuclear strike.

"The streets of the city are quite dangerous now," Chief of Police Volodymyr Tymoshko said, pleading with residents not to go outside. "Fighting continues due to small groups of the enemy who broke into the city."

In a video message posted on his Instagram account on Sunday, Zelenskyy described the night as "brutal" across the country. According to him, Russian forces were targeting residential buildings, kindergartens, and even ambulances.

Russia has denied that it has targeted civilians.

Kyiv will remain under strict curfew until 8 a.m. local time on Monday, complicating the task of assessing the intensity of the fighting, as residents were advised to avoid venturing out onto the streets and instead seek shelter.

Overnight, authorities reported clashes with Russian "sabotage groups."

The State Inspectorate for Nuclear Regulation of Ukraine said in a statement that Russian munitions hit a radioactive waste site, but the extent of the damage was unknown due to ongoing shelling and missile fire.

Vitali Klitschko, the mayor of Kyiv, later stated that there were no Russian troops in the city.

NBC News teams on the ground saw Ukrainians banding together to repel the invaders.

Richard Engel, the chief foreign correspondent, has seen people filling sandbags, guarding checkpoints, and standing armed at crossroads in every town and village he has passed through.

According to Russian state-run Interfax news agency, Russian forces on Sunday blocked the southern Ukrainian cities of Kherson, a port and gateway into the Black Sea, and Berdyansk on the Sea of Azov.

The pressure on strategic cities in Ukraine's south, including ports, appeared to be aimed at seizing control of the country's coastline.

While the Russian offensive appeared to be stymied by stronger-than-expected resistance from highly motivated Ukrainian armed forces, thousands of Ukrainians have fled to the country's western borders to escape the fighting.

According to the United Nations High Commissioner for Refugees, the number of Ukrainian refugees fleeing the country has risen to 368,000 and is expected to rise further.

Meanwhile, the Ukrainian government's human rights ombudsman, Lyudmyla Denysova, stated on Sunday that over 210 Ukrainian civilians have been killed and over 1,100 have been injured. Russia has not released casualty figures for its forces.