"We have not changed our own alerts, and we have not changed our own assessment in that front but we also need to be very clear-eyed about his own use of threats," White House spokeswoman Jen Psaki separately told MSNBC in an interview.

Russia's defence ministry on Monday said 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.

President Joe Biden held a call with U.S. allies and partners on Monday, as a U.S. official said a nuclear war must never be fought. Leaders from Canada, France, Germany, Italy, Japan, Poland, Romania and the United Kingdom along with NATO's chief and top officials from the European Union joined the call with Biden.

A White House official told Reuters that Putin was escalating the war with "dangerous rhetoric" and that Russia and the United States have long agreed that nuclear use would have devastating consequences.

"A nuclear war cannot be won, and must never be fought," the White House official said. "We think provocative rhetoric regarding nuclear weapons is dangerous, adds to the risk of miscalculation, should be avoided, and we will not indulge in it."

The official said the United States is assessing Putin's directive and at this time sees "no reason to change our own alert levels." Neither the United States nor NATO have any desire or intention for conflict with Russia, the official added.

The White House also pointed out Russia has signed onto joint statements affirming a nuclear war cannot be won and must never be fought.

Russian and Ukrainian officials met on the Belarusian border to discuss a ceasefire on Monday while invading Russian forces met with determined resistance from Ukrainian troops and civilians on a fifth day of conflict.

Russia's invasion of Ukraine has also triggered a Western economic and corporate response unprecedented in its extent and coordination.
The United States on Monday blocked Americans from engaging in any transactions involving Russia's central bank, dealing a crushing blow to the country's economy.