Geneva, Switzerland – US Secretary of State Antony Blinken will try to defuse a crisis with Moscow over Ukraine as he meets with Russian Foreign Minister Sergei Lavrov in Geneva on Friday.

Blinken's talks in the Swiss city got under way in the morning at the historic Hotel President Wilson on Lake Geneva. The two top envoys shook hands for the cameras in front of Russian and US flags before beginning their discussion.

The US and its allies are "committed to the path of diplomacy and dialogue," Blinken told Lavrov in a brief session open to reporters, ahead of a closed-door session scheduled to last two hours.

"But we are also committed, if that proves impossible and Russia decides to pursue aggression against Ukraine, to a united, swift and severe response."

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Blinken called it a "critical moment."

Both downplayed expectations, with Lavrov saying Russia did not expect any "breakthrough" but was eager to get "concrete responses" to its proposals for security guarantees, according to Russian news agencies.

The meeting follows a frenzied round of shuttle diplomacy in Europe as fears mount over Russia's build-up of forces along Ukraine's borders. Blinken was already in Kyiv and Berlin this week.

While the US and its Western allies are demanding a pull-back of the Russian troops, Moscow is demanding certain security guarantees and an end to the eastward expansion of the NATO military alliance.

Russia further objects to US military assistance to Ukraine, where a conflict has simmered in the east between Russian-backed separatists and Ukrainian forces since 2014.

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"All we're trying to do is to make sure to the best of our ability that ... Ukraine has the means to defend itself and that might perhaps deter further aggression by Russia," Blinken said on Thursday.

Efforts to ease the threat of large-scale Russian military operations against Ukraine have been in full swing for the past few weeks – including at NATO, the Organization for Security and Co-operation in Europe (OSCE), and direct talks with Moscow – but have so far produced no tangible results.