The United States and Russia open talks Sunday in Geneva on soaring tensions over Ukraine, with Moscow seeking a wide-ranging new security arrangement with the West but facing strong pressure to pull back troops.

The high-level discussions start a week of diplomacy in which Russia will meet with NATO and the Organization for Security and Cooperation in Europe (OSCE), with the US trying to assure European allies they will not be sidelined.

Russia since late last year has amassed tens of thousands of troops at the Ukrainian border and demanded guarantees that NATO will not expand eastward or set up further bases in the former Soviet Union.

The United States, to be represented by Deputy Secretary of State Wendy Sherman, agreed to talks even though it made plain that many of Moscow's proposals are non-starters. 

Originally scheduled to start on Monday, Sherman is now due to have a working dinner with Russian Deputy Foreign Minister Sergei Ryabkov on Sunday evening, said a State Department spokesperson.

Secretary of State Antony Blinken, dismissing Moscow's demands as "gaslighting", has insisted that talks will yield no progress so long as Russia has a "gun to Ukraine's head".

"We're prepared to respond forcefully to further Russian aggression. But a diplomatic solution is still possible and preferable if Russia chooses it," Blinken said Friday.

Russian President Vladimir Putin met his US counterpart Joe Biden in Geneva in June and agreed on regular "stability" talks between Sherman and Ryabkov, who will again lead the Russian delegation.

- 'Massive' retaliation -

In two phone calls to Putin, Biden has warned of severe consequences if Russia invades Ukraine.

Measures under consideration include sanctions on Putin's inner circle, cancelling Russia's controversial Nord Stream 2 pipeline to Germany or, in the most drastic scenario, severing Russia's links to the world's banking system.

A US official, speaking on condition of anonymity, warned that Washington would also send more troops to eastern NATO members such as Poland and the Baltics if Russia invaded.  

Europeans have showed solidarity, with EU foreign policy chief Josep Borrell visiting the frontline in Ukraine, although some nations are expected to hesitate at the strongest measures.

"Whatever the solution, Europe has to be involved," EU Commission chief Ursula von der Leyen said.

Russia insists it was deceived after the Cold War and understood that NATO would not expand.

Instead, the US-led alliance accepted most of the former Warsaw Pact nations and the three Baltic nations that were under Soviet rule.

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

Russia seized the Crimean peninsula and backs an insurgency in eastern Ukraine in which more than 13,000 people have died.

At a time that Russia is also intervening to shore up allies facing popular uprisings in Belarus and Kazakhstan, Moscow has insisted it wants concrete progress in talks with Washington.

Putin's foreign policy adviser Yury Ushakov warned after the call with Biden that the United States would make a "colossal mistake" if it went ahead with sanctions.

- 'Gigantic bluff'? -

NATO chief Jens Stoltenberg, meeting foreign ministers of the alliance on Friday, said there remained real risks of a Russian invasion.

But John Herbst, a former US ambassador to Ukraine, described the Russian troop build-up as a "gigantic bluff" by Putin to seek a negotiated agreement.

"They are trying to see if the Biden administration or Europe will blink," said Herbst, now at the Atlantic Council think tank.

"As long as the Biden administration remains at least as strong as it is now," he said, "it probably is enough to keep Putin from striking large into Ukraine, but I don't rule out something smaller."

Matthew Rojansky, director of the Kennan Institute at the Woodrow Wilson International Center for Scholars in Washington, said the Geneva talks were more about preventing the Ukraine crisis from accelerating than reaching a major deal.

"I think this is about pulling the pendulum back, if we can, towards thicker interaction and more effective diplomacy and communication -- not permanent and forever resolution of all problems."

While also downplaying the possibility of a full-scale invasion, Rojansky said the risks of the Russian build-up were real.

"There is the principle of Chekhov -- you put a loaded gun on the stage in Act One and it has to be fired by Act Three."

US ready to talk missiles, military exercises with Russia

The United States is ready to discuss with Russia the two countries' missile systems and military exercises, in talks that could begin as early as Sunday in Geneva, a senior White House official said Saturday. 

US and Russian diplomats are meeting in Switzerland to try to defuse tensions as Washington and Europe accuse Moscow of preparing a new invasion of its neighbor Ukraine. 

"There are some areas... where we think it might be possible to make progress," provided any promises are "reciprocal," the official said, giving some details on a conference call. 

"Russia has said it feels threatened by the prospect of offensive missile systems being placed in Ukraine... The United States has no intention of doing that. So this is one area where we may be able to reach an understanding if Russia is willing to make a reciprocal commitment," said the official, who requested anonymity. 

Moscow "has also expressed an interest in discussing the future of certain missile systems in Europe, along the lines of the INF Treaty (Intermediate-Range Nuclear Forces Treaty)" and "we're open to discussing this," the official said. 

Finally, Washington was ready to discuss the possibility of reciprocal restrictions on the size and scope of military exercises conducted by Russia as well as by the US and NATO, the source said. 

"We won't know until we get to these conversations starting tomorrow night whether Russia is prepared to negotiate seriously and in good faith," the official warned, saying the Russians and Americans would likely have an "initial conversation" on Sunday night before holding their "main meeting" on Monday. 

"We're going into these meetings with a sense of realism, not a sense of optimism," the source said, indicating that they would be "exploratory" and would not lead to firm promises. 

"I would not at all be surprised that Russian media begins to report perhaps even while the talks are still underway, that the US has made all manner of concessions to Russia. This is a deliberate attempt to create division among allies, in part by manipulating all of you," the official told reporters on the conference call.

US deputy secretary of state Wendy Sherman is to meet Russian deputy foreign minister Sergei Ryabkov for the Sunday evening talks, according to a State Department spokesperson.

Sherman will be accompanied by Lieutenant General James Mingus, the Joint Staff director of operations, and Ryabkov will attend the talks with Russian deputy defense minister Colonel General Aleksandr Fomin.