Kremlin spokesman Dmitry Peskov said Washington and NATO were escalating tensions through "information hysteria" and "concrete actions," adding that the risk of an offensive by Ukrainian troops against Moscow-backed separatists was "very high."

"We live in an aggressive environment," Peskov told reporters during a conference call, adding that President Vladimir Putin was taking "necessary measures" to protect the country.

NATO said earlier Monday it had put forces on standby and sent ships and fighter jets to bolster Europe's eastern defences as tensions soar over Russia's military buildup around Ukraine.

The West accuses Moscow of threatening to invade Ukraine by massing over 100,000 troops on its border. Russia denies it is planning an incursion and accuses NATO of advancing towards its borders.

Peskov said Monday that Ukraine was preparing an offensive in the east of the country where Kyiv troops have been fighting pro-Russia separatists since 2014.

"The Ukrainian authorities are concentrating a huge amount of forces and means on the border with the self-proclaimed republics," he said.

"The nature of this concentration speaks of preparations for an offensive," he said, adding the risk of such an operation now "is very high, higher than before."

Several rounds of talks between Russia and the West have so far failed to ease tensions.

The alliance is now preparing a proposal for further talks with Russia after the Kremlin issued a raft of demands that would see NATO stop Ukraine and Georgia joining and roll back its forces in eastern Europe.

NATO insists it will not negotiate over its "core principles" including defending all allies and allowing partners to chose their own path.

Peskov said Moscow was waiting to receive the written response from Washington and would decide on its further course of action after that.

He did not rule out further talks including new talks between Putin and US President Joe Biden.

"Nothing can be excluded if such a need arises," he said.