Putin puts nuclear forces on alert

STORY: Russian President Vladimir Putin put Russia's nuclear deterrent on high alert on Sunday, further raising the stakes amid his military assault on Ukraine.

But Ukrainian president Volodymyr Zelenskiy remained defiant, pledging to remain in the embattled capital, Kyiv.

Zelenskiy said on Sunday he'd agreed to new negotiations without preconditions with Moscow, even though he didn't hold out much hope.

"I do not really believe in the outcome of this meeting, but let them try, so that later not a single citizen of Ukraine has any doubt that I, as president, tried to stop the war."

Ukraine said it continued to hold off Russian forces from capturing Kyiv and Kharkiv, it's two largest cities.

Footage showed a Russian armored personnel carrier outside Kharkiv, where locals said at least one Russian soldier had been killed. An unexploded Russian warhead landed in a kindergarten playground.

In Kyiv, streets were deserted except for civil defense forces and the occasional ambulance. Plumes of smoke showed where Russian ordnance landed.

Nearly 400,000 Ukrainian civilians have fled the country into neighboring Poland and Romania.

The Russian invasion came after months of denials by Moscow that it planned to attack. Putin has called the attack a "special operation."

Western nations responded to the onslaught with an unprecedented array of sanctions on Russia's economy.

The European Union on Sunday shut all Russian planes out of its airspace.

Putin thrust an alarming new element into play when he ordered Russia's "deterrence forces" - which wield nuclear weapons - onto high alert.

A U.S. defense official said Washington was trying to assess what Putin's announcement meant in tangible terms, but that it increased the danger from any miscalculation.