NATO leaders agreed in the wake of Moscow's invasion of Ukraine in 2022 to massively ramp up the number of forces that alliance commanders can deploy within 30 days.

"The offers on the table from allies comfortably exceed the 300,000 that we set," the official said on condition of anonymity.

"Those are forces which allies have said to us, 'They are available to you as of now at that level of readiness'."

The push to have more troops ready to respond quickly is part of a broader overhaul of NATO's plans to stave off any potential Russian attack that was signed off at a summit last year.

Those plans laid out for the first time since the end of the Cold War what each member of the US-led alliance would be expected to do in case of an invasion by Moscow.

NATO commanders are currently trying to make sure they have the capabilities to execute those plans if needed.

But the alliance faces shortfalls in key weaponry such as air defences and longer-range missiles.

"There are capability gaps. There are things that we don't have enough of as an alliance at the moment and we need to tackle," the official said.