WASHINGTON, Feb 28 (AFP) - President Joe Biden said on Monday Americans should not worry about a nuclear war after Moscow put its nuclear deterrent on high alert amid a barrage of Western reprisals over Russia's invasion of Ukraine.

In response to a question about whether U.S. citizens should be concerned about a nuclear war breaking out, Biden said "no." He was responding to a shouted question at the end of an event at the White House.

White House officials said earlier in the day the United States sees no reason to change its nuclear alert levels at this time, a point press secretary Jen Psaki reiterated during her briefing with reporters.

"We are assessing President Putin's directive and at this time, we see no reasons to change our own alert levels," Psaki said.

Russia's defence ministry said on Monday its nuclear missile forces and Northern and Pacific fleets had been placed on enhanced combat duty, the Interfax news agency reported, in line with an order the previous day from President Vladimir Putin.