Kremlin spokesman Dmitry Peskov on Tuesday said Russia would only use nuclear weapons if its very existence were threatened, Tass news agency reported.

The comment, nearly four weeks after Russia sent its forces into Ukraine, came amid Western concern that the conflict there could escalate into a nuclear war.

Tass did not provide further detail.

President Vladimir Putin last month ordered Russia's nuclear forces to be put on high alert. In line with the order, Russia's defense ministry said on Feb. 28 that its nuclear missile forces and Northern and Pacific fleets had been placed on enhanced combat duty, the Interfax news agency reported.

United Nations Secretary General Antonio Guterres said on March 14: "The prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility."