Downing Street has said there is still a “window for diplomacy” to avert war over Ukraine after Joe Biden and Vladimir Putin tentatively agreed to a possible crisis summit.

Boris Johnson’s official spokesman said intelligence reports suggest Russia still intends to attack its neighbour, with the Kremlin’s plan beginning to play out.

However, after French President Emmanuel Macron worked to broker a meeting between the US and Russian leaders in a series of calls over the weekend, the spokesman said there is still a chance of a diplomatic resolution to the crisis.

“Intelligence we are seeing suggests Russia intends to launch an invasion and President Putin’s plan has in effect already begun,” the spokesman said.

“We are seeing elements of the Russian playbook we would expect to see in those situations starting to play out in real time.

“Crucially we still think there is a window for diplomacy. I think that is what we have seen in discussions over the weekend and we want to explore those.”

Earlier, following talks in Brussels with Nato Secretary-General Jens Stoltenberg, Foreign Secretary Liz Truss said an invasion still appears to be “highly likely”.

“Diplomacy must be pursued but a Russian invasion of Ukraine looks highly likely,” she tweeted.

“The UK and allies are stepping up preparations for the worst-case scenario. We must make the cost for Russia intolerably high.”

The latest warnings came after Western leaders repeatedly claimed that Moscow could be preparing a “false flag” operation to provide a pretext for an attack after massing more than 130,000 troops on Ukraine’s border.

Kremlin spokesman Dmitry Peskov said that while Mr Putin and Mr Biden could meet if they considered it necessary, no plans for a summit have been agreed.

“It’s premature to talk about specific plans for a summit. The meeting is possible if the leaders consider it feasible,” he said.

Russia’s ambassador to the UK, Andrei Kelin, said it is still too early to say whether a meeting between Mr Biden and Mr Putin would take place.

He told the PA news agency it would be a “good result” if sufficient progress was made in talks to allow the summit to go ahead.

But he said the West is “not very interested in resolving the core question, the issue of the enlargement of Nato, the open door policy”.

Further discussions are expected to take place on Thursday between US Secretary of State Antony Blinken and his Russian counterpart, Sergei Lavrov.

In a visit to the Commons, US House of Representatives speaker, Nancy Pelosi, suggested the Biden-Putin summit was contingent on the Lavrov talks being “successful”.

Meanwhile, Western officials said they were already seeing a number of Russian “provocations” including a supposed car bomb attack on one of the leaders of the Moscow-backed separatist rebels in the eastern of the country.

Officials said the Russians were continuing to move forces into the border region with high-end air defence capabilities, advanced aircraft and armoured vehicles deploying into forward locations over the past 72 hours.

It is estimated that Russia now has 110 tactical battalion groups in the area of operations with more still in transit.

About two thirds are said to be within 50km of the border, with half of those being “tactically deployed” having moved out of their staging areas.

One official said it represented a move “from being postured for military operations to being poised for military operations”.

Earlier, Mr Johnson signalled that the prospect of Mr Putin still being “willing to engage in finding a diplomatic solution” is a “welcome sign”.

But No 10’s readout of the Prime Minister’s own call with Mr Macron during the diplomatic flurry over the weekend did not appear overly optimistic about the prospect of a Russian climbdown.

The leaders “underscored the need for President Putin to step back from his current threats and withdraw troops from Ukraine’s border”, Downing Street said.

Mr Macron’s office said the Russian and US presidents had both “accepted the principle” of a summit, adding that the meetings “can only be held on the condition that Russia does not invade Ukraine”.

White House press secretary Jen Psaki said the US is “committed to pursuing diplomacy until the moment an invasion begins”, but noted that “currently, Russia appears to be continuing preparations for a full-scale assault on Ukraine very soon”.

Russia and its ally Belarus said they are extending war games on Belarusian territory which would offer a convenient staging post on Ukrainian capital Kyiv, less than 50 miles south of the border with Belarus.

Heavy shelling in Ukraine continued on Monday in the heightened tension between Ukrainian forces and Russian-backed separatist rebels in the Donbas region.