U.N. Secretary-General Antonio Guterres said on Tuesday that any Russian troops deployed to eastern Ukraine would not be "peacekeepers" as Moscow has asserted and rejected President Vladimir Putin's claim that a genocide of ethnic Russians was being committed there.

The United States has dismissed Russia's justification to deploy troops as "peacekeepers" as "nonsense," accusing Moscow of building a pretext for war.

"When troops of one country enter the territory of another country without its consent, they are not impartial peacekeepers. They are not peacekeepers at all," Guterres told reporters at the United Nations.

He also said Russia had violated the territorial integrity and sovereignty of Ukraine by recognizing two separatist regions in eastern Ukraine - the self-proclaimed Donetsk People's Republic and the Luhansk People's Republic - as independent.

Putin on Tuesday ordered his defense ministry to despatch Russian forces to "perform peacekeeping functions" in eastern Ukraine. He later told reporters that he did not say Russian troops would go there immediately.

Guterres said he was "concerned about the perversion of the concept of peacekeeping."

Putin said last week that Russia considered the treatment of ethnic Russians in eastern Ukraine to be genocide.

"I do not think it is the case," Guterres said when asked about Putin's assertion.

Under international law, genocide is an intent to destroy, in whole or in part, a national, ethnical, racial or religious group.

This includes through killings; serious bodily or mental harm; inflicting conditions of life calculated to bring about its physical destruction; imposing measures to prevent births; forcibly transferring children of the group to another group.

Tensions between Russia and the West have escalated over U.S. accusations that Moscow has deployed up to 150,000 troops near Ukraine's borders. Russia has denied it wants to invade Ukraine and accuses the West of hysteria.

"We need restraint and reason. We need de-escalation now," Guterres said. "I urge all to refrain from actions and statements that would take this dangerous situation over the brink."