Russia said Thursday that the United States was failing to address its main security concerns over Ukraine but left the door open to further talks to ease tensions.

Relations between Russia and the West have reached their lowest point since the Cold War after Moscow deployed tens of thousands of troops on the border of pro-Western Ukraine, raising fears of an invasion.

Russia denies any plans to invade but last month put forward demands for wide-ranging security guarantees from the West, including that Ukraine never be allowed to join the US-led NATO military alliance.  

The United States on Wednesday delivered a reply in co-ordination with NATO allies, rejecting any ban on Ukraine, but offering what it called a new "diplomatic path" out of the crisis.

In its first reaction to the reply, the Kremlin was unimpressed.

"It cannot be said that our views were taken into account, or that a readiness to take our concerns into account was demonstrated," Kremlin spokesman Dmitry Peskov told reporters.

"Let's not rush into assessments, it takes time to analyse," he said, adding that the documents were with President Vladimir Putin.

Foreign Minister Sergei Lavrov said Moscow's chief concern -- the potential for Ukraine to join NATO -- had been ignored, but that it would be possible to move forward on other issues.

"There was no positive response to the main question," Lavrov said in a statement, but "there is a response which gives hope for the start of a serious conversation on secondary questions".

NATO's door is open

Secretary of State Antony Blinken said on Wednesday that he would speak again in the coming days to Lavrov.

Blinken said the reply, which would remain confidential, "sets out a serious diplomatic path forward should Russia choose it".

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.

"NATO's door is open, remains open, and that is our commitment," Blinken said.

Russia, which has a fraught historical relationship with Ukraine, has fuelled an insurgency in the former Soviet republic's east that has killed more than 13,000 people since 2014.

Russia that year also seized Crimea after the overthrow of a pro-Russian government in Kyiv.

The United States has warned of severe and swift consequences if Russia invades, including possible personal sanctions on Putin, and NATO has put 8,500 troops on standby.

"While we are hoping for and working for a good solution -- de-escalation -- we are also prepared for the worst," NATO Secretary General Jens Stoltenberg said on Wednesday.

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

More talks in two weeks 

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.

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.

Ukrainian Foreign Minister Dmytro Kuleba on Thursday hailed the talks as a positive sign.

"The good news is that advisors agreed to meet in Berlin in two weeks, which means that at least for the next two weeks, Russia is likely to remain on a diplomatic track," Kuleba told reporters in Copenhagen.

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

The United States on Wednesday again encouraged its citizens to leave Ukraine, warning an invasion could be imminent.

But Ukraine's government, hoping to prevent panic, has played down the dangers and sought to offer ways out.

Kuleba told reporters on Wednesday that the Russian troops posed "a threat to Ukraine" but that the numbers deployed were "insufficient for a full-scale offensive".