WASHINGTON, 27 Feb (REUTERS) -- The United States and NATO on Sunday (Feb 27) condemned Russian President Vladimir Putin's order to put his nuclear forces on high alert as dangerous and unacceptable, while the White House said it has not ruled out imposing new sanctions on Russia's energy sector.

In issuing the order to prepare Russia's nuclear weapons for increased readiness for launch, Putin cited "aggressive statements" from NATO allies and widespread sanctions imposed by Western nations.

The US ambassador to the United Nations, Linda Thomas-Greenfield, told CBS's Face the Nation programme that Putin's actions have escalated the conflict and were "unacceptable".

Thomas-Greenfield said the United States was "continuing to look at new and even harsher measures against the Russians".

At the Pentagon, a senior US defense official also described Putin's nuclear order as an escalation and said it was "putting in play forces that, if there's a miscalculation, could make things much, much more dangerous".

The United States is trying to determine what Putin's order means "in tangible terms," the official said, speaking on condition of anonymity.

On CNN's State of the Union programme, NATO Secretary-General Jens Stoltenberg called Putin's nuclear order "aggressive" and "irresponsible".