Downing Street has warned that Russian President Vladimir Putin’s plan for invading Ukraine has effectively “already begun”.
Number 10 suggested that Russia’s “playbook” for launching military action against its neighbour had begun to “play out in real time”.
But a “window for diplomacy” still remains to prevent war in Europe, Downing Street added.
Live updates on Ukraine crisis as satellite images reveal new Russia deployments
Earlier on Monday, Foreign Secretary Liz Truss held talks with NATO secretary general Jens Stoltenberg in Brussels about the continuing crisis.
After their meeting, Ms Truss warned that a Russian invasion of Ukraine “looks highly likely” and that the UK and NATO allies were “stepping up preparations for the worst case scenario”.
The foreign secretary’s downbeat assessment came despite Mr Putin and US President Joe Biden agreeing “in princple” to meet at a crisis summit, following a flurry of diplomatic calls over the weekend.
Asked about Ms Truss’s comments, the prime minister’s official spokesman said: “That is our assessment.
“The intelligence we’re seeing suggests Russia intends to launch an invasion and that President Putin’s plan has already begun, in effect.
“We’re seeing elements of the Russian playbook that we would expect to see in those certain situations start to play out in real time.
“We’ve been working with NATO allies to call that out when we see it.
“The intelligence we have suggests that they still intend to launch an invasion and we stand ready to act as needed.
“But, crucially, we still think there is a window for diplomacy – as we’ve seen with discussions over the weekend – and we want to explore those.”
French President Emmanuel Macron has been credited with brokering the possible meeting between Mr Putin and Mr Biden in a series of phone calls on Sunday night.
US secretary of state Antony Blinken and Russian foreign minister Sergey Lavrov are due to lay the groundwork for the summit at a meeting on Thursday.
However, Russia’s ambassador to the UK said it was too soon to say whether a Putin-Biden meeting would be a success.
“I do believe that it’s a bit early now to say what’s going to happen,” Andrei Kelin told the PA news agency.
Mr Kelin also reiterated Moscow’s message that Russia would retaliate should the UK impose new sanctions.
UK ministers recently gained powers to impose tough new sanctions on Russian oligarchs and businesses.
But Mr Kelin said the efforts to prepare new sanctions were “a huge overreation to the unusual circumstances”.
“We will of course find ways to respond,” he added.
UK government minister Paul Scully told Sky News on Monday that, despite recent claims from Moscow that Russian troops were being withdrawn from Ukraine’s borders, there were 7,000 more troops in the area than there were a few days ago.
“That’s why we’ve got to continue to be vigilant,” he said as he warned of a “horrendous” loss of life should Mr Putin ignore a diplomatic route out of the crisis.
Heavy shelling has increased in recent days along the line of contact between Ukrainian forces and Russian-backed separatist rebels in Ukraine’s eastern industrial heartland of Donbass.