“The plans involve extensive movement of 100 battalion tactical groups with an estimated 175,000 personnel, along with armour, artillery and equipment.”

This assessment was based partly on photos taken by spy satellite that “show newly arrived units at various locations along the Ukrainian border over the last month,” the official said.

The newly published intelligence came after Secretary of State Antony Blinken told European allies this week that the US has “evidence that Russia has made plans for significant aggressive moves against Ukraine”.

Speaking during a trip to Europe, Mr Blinken said it was unclear whether Vladimir Putin, the Russian president, had reached a final decision to invade.

“We do know that he is putting in place the capacity to do so on short order should he so decide,” he said, a day before meeting Sergei Lavrov, the Russian foreign minister. “We must prepare for all contingencies.”

Mr Putin has dismissed reports of another invasion as rumors, while Mr Lavrov has warned against US military activity near Russian borders. “The nightmare scenario of military confrontation is returning,” Mr Lavrov said this week.

The Kremlin, alarmed by the expansion of Nato into former Warsaw Pact countries, is seeking guarantees that Ukraine will not be allowed to join the Western alliance.

But Mr Biden has rejected the demand: “I don’t accept anyone’s red line,” he said.