The United States and its European allies closed ranks as they prepared to meet Russian envoys Wednesday at NATO for talks to calm tensions on the Ukraine border, with both sides refusing to give ground.

Russia's massive troop build-up around already partially-occupied Ukraine has forced Washington to engage with Moscow to head off fears of an all-out military confrontation.

But, after an initial round of US-Russia talks in Geneva on Monday proved inconclusive, the next round of dialogue was to move on Wednesday to NATO headquarters in Brussels.

Ahead of the meeting, US negotiator Deputy Secretary of State Wendy Sherman rallied Western allies, some of whom were worried they might be sidelined.

"It's too early to tell whether the Russians are serious about the path to diplomacy or not, or if they're prepared to negotiate seriously –- we are," White House spokeswoman Jen Psaki said Tuesday.

US President Joe Biden's spokeswoman insisted: "NATO'S relationship with Ukraine is a matter only for Ukraine and the 30 NATO allies, not for other countries to determine."

But Washington's European allies are keen not to be sidelined, as President Vladimir Putin's Kremlin seeks to roll back what it sees as the West's post-Cold-War encroachment on its turf.

"There's no reason to be optimistic," a senior European diplomat told AFP. "But the Russians are seriously engaged on the diplomatic track." 

Ukraine's President Volodymyr Zelensky on Tuesday reiterated his demand that France and Germany host a new international summit between Moscow and Kyiv to end the conflict.

The French presidency said the Kremlin had agreed for France, Germany, Russia and Ukraine to hold such talks "by the end of January".

And Zelensky's spokesman Sergiy Nykyforov welcomed "the intent and efforts of the United States and Russia, and NATO and Russia to reduce tensions and resolve all mutual issues at the negotiating table."

Ukraine's Foreign Minister Dmytro Kuleba said "unity" was key against what he described as "Russian ultimatums".

- 'Moment of truth' -

On Tuesday, Sherman was at NATO headquarters to brief European allies.

She brought NATO Secretary General Jens Stoltenberg up to date on her Geneva talks with Russia's Deputy Foreign Minister Sergei Ryabkov, and then met NATO ambassadors.

"The United States is committed to working in lockstep with our allies and partners to urge de-escalation and respond to the security crisis caused by Russia," she said.

After more than seven hours of negotiations in Geneva on Monday, the Russian and US officials both offered to keep talking, though there was no breakthrough.

At NATO, Russia will be represented by Deputy Foreign Minister Alexander Grushko, who has described the meeting as "a moment of truth" in Russia-NATO relations.

Moscow's demands include a concrete guarantee that Ukraine will not be allowed to join NATO.

The allies have long insisted that NATO membership is a matter for sovereign states to decide for themselves and on Tuesday once again vowed to preserve their open-door policy.

They also threatened massive economic and financial sanctions against Moscow if its huge troop build-up on Ukraine's frontiers and in already Russian-occupied Crimea turns into a new invasion.

The US State Department's number three diplomat, Victoria Nuland, said "the Kremlin has to justify to the Russian people why it is stoking a potentially very bloody and costly conflict for Russia.

"We will respond with massive economic measures, including those that have not been used before, and will inflict very significant costs on Russia's economy and its financial system."

But, speaking after Monday's talks and ahead of his return to Brussels, Grushko said Russia would demand a comprehensive response from the alliance to its demands.

"We will push for a concrete, substantive, article-by-article reaction to the Russian draft agreement on guarantees," he added.

- Some demands 'non starters' -

Sherman said Russia offered no proof it would not invade or any explanation for why it has deployed some 100,000 troops towards the Ukrainian border.

She had offered limited de-escalation moves, with Washington and Moscow to agree on reciprocal limits to missile batteries and exercises.

The new US ambassador to NATO, Julianne Smith, told reporters the talks could discuss "reciprocal restrictions on exercises". 

She described the broad themes of Wednesday's talks as "risk reduction, transparency, arms control and various ways in which we communicate with each other."

Russia has put intense pressure on Ukraine since 2014, after a revolution overthrew a government that had sided with the Kremlin against moving closer to Europe.

Russia has seized and annexed the Ukrainian region of Crimea and Moscow backs an insurgency in eastern Ukraine in which more than 13,000 people have died.