In recent weeks, Russia has been amassing large numbers of troops on the Ukraine border - something which Western countries have seen as preparation for a possible invasion. Russia denies this.

Mr Blinken said the US response made its "core principles" clear, including Ukraine's sovereignty and its right to choose to be part of security alliances such as Nato.

"There should be no doubt about our seriousness of purpose when it comes to diplomacy, and we're acting with equal focus and force to bolster Ukraine's defences and prepare a swift united response to further Russian aggression," he said.

"It remains up to Russia to decide how to respond," he added. "We're ready either way."

The secretary of state said the US had sent three shipments of military "assistance" this week - including Javelin missiles and anti-armour weaponry, along with hundreds of tonnes of ammunition and equipment.

Mr Blinken also denied any rift or difference of opinion between the US and its European allies. Nato, he said, had prepared its own set of proposals which "fully reinforces ours and vice versa".

But the US document will not be made public.

"Diplomacy has the best chance to succeed if we provide space for confidential talks," the Secretary of State said.

Nato Secretary General Jens Stoltenberg said the alliance's document had also been delivered to Moscow, and while he was willing to listen to Russia's concerns, all nations had the right to choose their own security arrangements.

Russian Foreign Minister Sergei Lavrov, however, said earlier on Wednesday that Mr Stoltenberg had "lost touch with reality", when asked about Nato boosting its presence near Russia's borders.