KYIV, Feb 28 (Reuters) - President Volodymyr Zelenskiy urged the West to consider a no-fly zone for Russian aircraft over Ukraine on Monday after Moscow bombarded the country's second largest city, drawing new sanctions by the United States and its allies.

Russia faces increasing international isolation over its invasion of Ukraine, and hours of negotiations between the two sides on Monday failed to reach a breakthrough to halt the fighting.

Ukrainian officials said Russian attacks in Kharkiv, a city of 1.4 million people, had killed civilians, including children. In a video address, Zelenskiy said it was time to block Russian missiles, planes and helicopters from Ukraine's airspace.

"Fair negotiations can occur when one side does not hit the other side with rocket artillery at the very moment of negotiations," Zelenskiy said. He did not specify how and by whom a no-fly zone would be enforced.

The United States has ruled out sending troops to fight Russia and officials have voiced concern about further escalating tensions between the world's two biggest nuclear powers.

"A no-fly zone would require implementation," White House Press Secretary Jen Psaki told reporters.

Such a move would require "deploying U.S. military to enforce, which would be... potentially a direct conflict, and potentially a war with Russia, which is something we are not planning to be a part of."

Western nations, united in condemnation of Russia's assault, have hit Russia with sanctions, with targets including President Vladimir Putin and his confidants.

But Putin has shown no sign of reconsidering the invasion he unleashed on Russia's neighbour last Thursday in an attempt to redraw Europe's security map and pull Ukraine firmly into its orbit.

The Russian leader put Russia's nuclear forces on high alert on Sunday, though a senior U.S. defence official said Washington had still not seen any "muscle movement" following Putin's announcement.
Asked at the White House on Monday if Americans should worry about nuclear war, U.S. President Joe Biden said: "No."

But in a sign of souring relations, the United States expelled 12 Russian diplomats at the United Nations, citing national security concerns. Russia described the move as "hostile."