US Deputy Secretary of State Wendy Sherman (left) is greeted by Delphine Pronk prior to a meeting in Brussels. AP

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.

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Meanwhile, Russia is set to lay out its demands for security guarantees in Europe to NATO's 30 allies on Wednesday, following intense talks with the United States in Geneva that showed the two sides have major differences to bridge.

Wendy Sherman (left) speaks with Delphine Pronk in Brussels. AP

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.

The NATO-Russia Council at allied headquarters in Brussels is part of a broader effort to defuse the worst East-West tensions since the Cold War, triggered primarily by a confrontation over Ukraine, which the United States says Russia is planning to invade.

The NATO-Russia Council at allied headquarters in Brussels.

Moscow dismisses such claims, though it is massing troops near the Ukrainian border.

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."