Russian frontline commanders have been given orders to begin final preparations for an attack on Ukraine, according to a US official.

The intelligence is said to have informed the White House’s belief that Moscow intends to commence a “full-scale assault” very soon.

In a potential breakthrough, Joe Biden and his Russian counterpart Vladimir Putin have agreed in principle to attend a summit on the crisis.

The talks would be brokered by the French, centre on “security and strategic stability in Europe”, and offer a possible path out of one of the most dangerous crises for the continent in decades.

Washington said the meeting would only take place if an invasion hasn’t happened, and White House Press Secretary Jen Psaki warned: “We are also ready to impose swift and severe consequences should Russia instead choose war.”

Further details on what would be discussed at the summit are set to be worked out by US Secretary of State Antony Blinken and Russian Foreign Minister Sergei Lavrov when they meet on Thursday – but again, these talks are on the condition that the Kremlin doesn’t invade Ukraine.

Yesterday, Russia rescinded on an earlier pledge to pull thousands of troops back from Ukraine’s northern border – with US leaders claiming this move puts Moscow one step closer to an invasion.

Military exercises in Belarus involving an estimated 30,000 Russian soldiers had been due to end on Sunday, but these drills have now been extended.

This continued deployment has raised concerns that the personnel could be used to sweep down on the Ukrainian capital of Kyiv, which is less than a three-hour drive away.

Read more: Kyiv volunteers stock emergency shelters amid warnings of imminent invasion

In other developments, the US embassy in Moscow has urged Americans in Russia to take greater caution – and “have evacuation plans that do not rely on US government assistance”.

Separatist leaders are also continuing to evacuate civilians out of the eastern Ukrainian regions of Luhansk and Donetsk as shelling intensifies.

Leaders in Ukraine and the West fear these areas could be the flashpoint in igniting conflict.

Read more: As warnings of war grow, fear is also being felt in neighbouring Poland

Early on Monday, Russian-backed rebels claimed two civilians were killed in shelling by Kyiv government forces, according to a report by the RIA news agency.

In an interview on Sunday, Vladimir Putin’s spokesman Dmitry Peskov said: “When tension is escalated to the maximum, as it is now, for example, on the line of contact, then any spark, any unplanned incident or any minor planned provocation can lead to irreparable consequences.”

Ukraine’s border is currently surrounded on three sides by an estimated 150,000 Russian soldiers – as well as warplanes and equipment.

Read more: How big is Russia’s military – and how does it compare with Ukraine?

Yesterday, the US secretary of state told CNN: “Everything we are seeing suggests this is dead serious … Until the tanks are actually rolling, and the planes are flying, we will use every opportunity and every minute we have to see if diplomacy can still dissuade President Putin from carrying this forward.”

Boris Johnson spoke with French President Emmanuel Macron by phone on Sunday, and agreed that the coming week will be “crucial for diplomacy”.

A Downing Street spokeswoman said the prime minister stressed that “Ukraine’s voice must be central in any discussions”.