The proposals, set out in letters by NATO and the United States last month in response to Russian demands, remain firm on insisting that Ukraine and any other sovereign country has a right to apply to join the alliance.

But the reported US response -- posted to the Spanish daily's website -- suggests "reciprocal commitments by both the United States and Russia to refrain from deploying offensive ground-launched missile systems and permanent forces with a combat mission in the territory of Ukraine".

Both the US and NATO documents urge Russia to restore diplomatic ties with the alliance and to renew and renegotiate nuclear missile control treaties with the United States.

Moscow is urged to re-engage with the NATO-Russia council, a diplomatic body "offering dialogue and partnership in place of conflict and distrust".

A NATO official refused to confirm the text, saying: "We never comment on alleged leaks."

Kremlin spokesman Dmitry Peskov said Moscow was aware of the report but added: "We didn't publish anything, and I don't want to comment on this."

But later, US State Department spokesman Ned Price said the El Pais report accurately described the US position.

"I have seen nothing to suggest these documents are not authentic," he said, while insisting that Washington did not leak them.

- Huge Russian force -

President Vladimir Putin's government has deployed a huge force -- more than 100,000 strong -- on its territory near the Ukraine border and in Crimea, a Ukrainian region that Russia annexed in 2014.

The Kremlin has demanded that NATO guarantee that Kyiv never be allowed to join NATO and that the alliance withdraw forces from eastern member states that were Soviet allies or republics during the Cold War.

The western allies have dismissed calls to slam shut NATO's door, but the leaked letters call for "meaningful arms control discussions and dialogue with Russia on mutual transparency and confidence-building measures".

"No other partner has been offered a comparable relationship or a similar institutional framework," the allies said of the NATO-Russia council, in the letter released by El Pais.

"Yet Russia has broken the trust at the core of our cooperation and challenged the fundamental principles of the global and Euro-Atlantic security architecture," it says.

The US document stresses: "We are ready to consider arrangements or agreements with Russia on issues of bilateral concern, to include written, signed instruments, to address our respective security concerns."

It suggests renewing the US-Russian Strategic Stability Dialogue on arms control agreements to "limit ground-based intermediate and shorter-range missiles and their launchers".

But it repeats Washington's warning that Russia is already in breach of the now suspended 1987 Intermediate-Range Nuclear Forces Treaty, which the US withdrew from in 2019, accusing Moscow of deploying a banned type of missile.

And it warns: "Further Russian increases in force posture or further aggression against Ukraine will force the United States and our Allies to strengthen our defensive posture."