Russia has sharply raised the stakes in a showdown with the West over Ukraine, with a top diplomat saying he would not exclude a Russian military deployment to Cuba and Venezuela if tensions with the United States mount.

Deputy foreign minister Sergei Ryabkov, who led the Russian delegation in Monday's talks with the US in Geneva, said in televised remarks that he would neither confirm nor exclude the possibility that Russia could send military assets to Cuba and Venezuela.

The negotiations in Geneva and Wednesday's Nato-Russia meeting in Vienna failed to narrow the gap on Moscow's security demands amid a build-up of Russian troops near Ukraine.

While Moscow demanded a halt to Nato expansion, Washington and its allies firmly rejected them as a non-starter.

Speaking in an interview with Russian RTVI TV, Mr Ryabkov noted that "it all depends on the action by our US counterparts", adding that President Vladimir Putin has warned that Russia could take military-technical measures if the US provokes Moscow and turns up military pressure on it.

Mr Ryabkov said a refusal by the US and its allies to consider the key Russian demand for guarantees against the alliance's expansion to Ukraine and other ex-Soviet nations raises doubts about continuing the talks.

Kremlin spokesman Dmitry Peskov noted "some positive elements and nuances" during the talks, but described them as "unsuccessful" because of stark disagreements on Russia's key demands.

"The talks were initiated to receive specific answers to concrete principal issues that were raised, and disagreements remained on those principal issues, which is bad," he said in a conference call with reporters.

Mr Peskov warned of a complete rupture in US-Russian relations if proposed sanctions targeting Mr Putin and other top civilian and military leaders are adopted.

The measures, proposed by Senate Democrats, would also target leading Russian financial institutions if Moscow sends troops into Ukraine.

Mr Peskov criticised the proposals as an attempt to up the pressure on Moscow during the talks, saying it would not work.

"It concerns sanctions, which taking into account the inevitable adequate response, effectively amount to an initiative to rupture relations," he warned, adding that Russia will respond in kind to protect its interests.

The talks come as an estimated 100,000 combat-ready Russian troops, tanks and heavy military equipment are massed near Ukraine's eastern border.

The build-up has caused deep concerns in Kyiv and the West that Moscow is preparing for an invasion.

Russia denies that it is pondering an invasion and in turn accuses the West of threatening its security by positioning military personnel and equipment in Central and Eastern Europe.

Mr Peskov rebuffed the West's calls for Russia to help de-escalate tensions by pulling back troops from areas near Ukraine, noting that the country is free to move them wherever it deems necessary on its own territory.

"It's hardly possible for Nato to dictate to us where we should move our armed forces on the Russian territory," he said.

Mr Peskov underscored that Russia is ready to continue the talks but wants them to produce results.

"There will be no deficit of a political will to continue the negotiations," he said.

Tensions revolving around Ukraine and Russia's demands on the West again appeared on the table at Thursday's meeting of the Organisation for Security and Co-operation in Europe (OSCE) in Vienna.

Polish foreign minister Zbigniew Rau, who assumed the position of the OSCE chairman-in-office, noted in his opening speech that "the risk of war in the OSCE area is now greater than ever before in the last 30 years".

"For several weeks, we have been faced with the possibility of a major military escalation in Eastern Europe," he said.

"We have recently heard a demand for security guarantees related to an important part of the OSCE area and the renewed discourse about spheres of influence. All these aspects require a serious international assessment and a proper reaction."

Mr Rau emphasised the need to "focus on a peaceful resolution of a conflict in and around Ukraine ... in full respect of the sovereignty, territorial integrity and unity of Ukraine within its internationally recognised borders".

In 2014, Russia annexed Ukraine's Crimean Peninsula after the ousting of its Moscow-friendly leader and threw its weight behind a separatist insurgency in the country's east, where more than seven years of fighting has killed more than 14,000 people.

A 2015 peace deal brokered by France and Germany has helped end large-scale battles, but frequent skirmishes have continued and efforts to negotiate a political settlement have failed.