US Secretary of State Antony Blinken and Russian Foreign Minister Sergei Lavrov met for talks in Geneva on Friday amid mounting fears that Russia could be about to invade Ukraine.
"This is a critical moment," Mr Blinken said in his opening remarks.
The US and Russia "don't expect to resolve our differences here today", he added, but hoped to test whether diplomacy was still a viable option.
Mr Lavrov said Russia was not expecting a breakthrough in the key talks.
Moscow has 100,000 troops near its borders with Ukraine, but denies planning to invade.
Across the table in a luxury Swiss hotel, Mr Blinken warned his Russian counterpart of a "united, swift and severe" response if Russia did take that step.
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