US Secretary of State Antony Blinken attends a press conference. File photo

The US has made no concessions to the main Russian demands over Ukraine and NATO in a long-awaited written response delivered to Russia on Wednesday in Moscow, Secretary of State Antony Blinken said.

Blinken said the US response, delivered to the Russian Foreign Ministry by US Ambassador to Russia John Sullivan, gave up no ground on "core principles” such as NATO’s open-door membership policy and the alliance’s military presence in Eastern Europe.

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Moreover, the United States on Wednesday rejected Russia's key demand to bar Ukraine from NATO and said it believed Moscow was ready to invade but offered what it called a new "diplomatic path" out of the crisis.

Russia has placed more than 100,000 troops near Ukraine's border and ratcheted up war games in the region.

Moscow denies it is planning an assault, but the United States and its NATO allies fear that Russia is planning for war — and are making preparations of their own.

Meanwhile, China's Foreign Minister Wang Yi said on Thursday that all parties involved in the Ukraine issue should remain calm and refrain from actions that stir tension and hype up the crisis, as he held talks with US Secretary of State Antony Blinken.

Russia has demanded that NATO pull back troops and weapons from eastern Europe and bar its neighbour Ukraine, a former Soviet state, from ever joining.

China's Foreign Minister Wang Yi speaks during a meeting. File photo

Secretary of State Antony Blinken said he would speak again in the coming days to Russian Foreign Minister Sergei Lavrov, as a separate initiative by France brought a promise by Moscow at least to keep talking to Ukraine's government.

One month after Russia put forward sweeping security proposals, having sent tens of thousands of troops to Ukraine's border, the United States delivered a reply in co-ordination with NATO allies and said it was ready for any eventuality.

"It sets out a serious diplomatic path forward should Russia choose it," Blinken told reporters of the US response, which he said would remain confidential.

He renewed an offer on "reciprocal" measures to address mutual security concerns, including reductions of missiles in Europe and transparency on military drills and Western aid to Ukraine.

But he made clear that the United States would not budge on Russia's core demand that Ukraine never be allowed to join NATO, the US-backed military alliance.

"From our perspective, I can't be more clear — NATO's door is open, remains open, and that is our commitment," Blinken said.

French-led talks

In another bid to defuse tensions, senior Russian and Ukrainian officials met for eight hours in Paris with representatives of France and Germany.

Dmitry Kozak, the Kremlin deputy chief of staff, said the talks were "not simple" but that another round would take place in two weeks in Berlin.

Kremlin's deputy chief of staff Dmitry Kozak gives a press conference in Paris on Wednesday. AFP

France said after the so-called Normandy Format talks that the envoys committed to a fragile July 2020 ceasefire in eastern Ukraine between government forces and pro-Moscow separatists.

"We need a supplementary pause. We hope that this process will have results in two weeks," Kozak said.

An aide to French President Emmanuel Macron stressed that the talks had been about resolving the separatist fighting in eastern Ukraine, not the threat of a Russian invasion.

But "the question was whether the Russians wanted to signal a thaw," he said, adding that the "difficult" discussions had ultimately resulted in something positive.

"In the current circumstances, we received a good signal," he said.

France and Germany have joined the United States in warning Russia against an invasion but have been less direct about sanctions.

Germany's new coalition government has sent mixed signals on whether it would sever the soon-to-open Nord Stream 2 pipeline from Russia, which will circumvent Ukraine to provide gas to Europe's largest economy.

US President Joe Biden, who spoke with European leaders by video-conference on Tuesday, said that any Russian military attack on Ukraine would trigger "enormous consequences" and could even "change the world."

Kremlin spokesman Dmitry Peskov, while brushing off the impact, warned that attempts to punish Putin personally would be "destructive."