US Secretary of State Antony Blinken said the "ball is in Russia's court", as he added: "We're ready either way."

Russia has massed about 100,000 troops near the Ukrainian border in recent weeks and is holding military drills at multiple locations in Russia, with similar exercises planned in neighbouring Belarus, which also borders Ukraine.

Moscow has denied it is planning an assault on Ukraine.

Mr Blinken said the US response made no concessions to Moscow's main demand that Ukraine be prevented from ever joining NATO.

Washington has also made it clear that Russia's push for the Western alliance to pull back troops and weapons from eastern Europe is also a non-starter.

The top diplomat said the document stressed the US is standing by its often-stated positions - "there is no change, there will be no change".

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He said the document, handed to officials in the Russian capital by the US ambassador, outlines a diplomatic path forward if the Kremlin chooses it, as America tries to avert a military offensive.

It contains a "principled and pragmatic" evaluation of Moscow's concerns and reflects that the US is open to dialogue.

US ready to discuss arms control

And Mr Blinken said the US is ready to discuss other topics such as arms control and confidence-building measures.

He added Ukraine and Washington's European allies were closely involved in drawing up the document which he said would not be made public.

He said: "Putting things in writing is... a good way to make sure we're as precise as possible, and the Russians understand our positions, our ideas, as clearly as possible. Right now, the document is with them and the ball is in their court."

The diplomat said he expects to speak again to Russian Foreign Minister Sergei Lavrov within days to find out what Russia thinks of the response.

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Some 8,500 US troops have been put on heightened alert to go to eastern Europe to bolster NATO forces, but not to Ukraine itself.

NATO currently has about 4,000 troops in multinational battalions in Estonia, Lithuania, Latvia and Poland, backed by tanks, air defences and intelligence and surveillance units.

Read more: Inside tank base that provides Ukraine's best hope of defence against Russian invasion

NATO secretary-general Jens Stoltenberg has called on Russia to immediately de-escalate the situation.

He told reporters: "NATO firmly believes that tensions and disagreements must be resolved through dialogue and diplomacy."

He also repeated his call for Moscow and the pact to re-establish their respective offices in Brussels and Moscow, and to use military communication channels to promote transparency and reduce risks.

Earlier, the Kremlin warned it would quickly take "retaliatory measures" if the US and its allies reject its security demands and continue their "aggressive" policies.

Russia has been carrying out new military drills on land and on the Black Sea and moved more paratroopers and fighter jets to Belarus, north of Ukraine, for what it describes as joint exercises there next month.