LONDON: A Russian invasion of Ukraine is highly likely, could be imminent and would pose a threat to Europe's wider stability that emboldens aggressors around the world, Britain's Foreign Secretary Liz Truss said on Tuesday.

Russia has more than 100,000 troops massed near the border of Ukraine. Russian political leaders deny Western accusations that it is planning to invade, but say it could take unspecified "military-technical" action unless a range of demands are met, including barring Kyiv from ever joining the NATO alliance.

British Prime Minister Boris Johnson and US President Joe Biden on Monday agreed in a call on Monday that there was a crucial window for diplomacy, but Truss said an invasion could be imminent, and Russian troops could reach Kyiv "very, very quickly."

NATO to consider new troops on southeastern flank amid Russian threat

"This is ... about the wider stability of Europe," she told Sky News. "And it's about wider global stability, and the message that we give to aggressors and we have to give the message to Vladimir Putin that there can be no reward for aggression."

Truss echoed politicians in the United States who have warned that a so-called "false flag" operation could be used by Moscow to trigger a conflict.

"It is still the case that an invasion could be imminent, and it is highly likely," she said.