US Secretary of State Antony Blinken and Russian Foreign Minister Sergei Lavrov met for talks in Geneva.
“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.
Speaking afterwards, Mr Lavrov described the talks as open and useful.
Moscow has 100,000 troops near its borders with Ukraine, but denies planning to invade.
Mr Blinken warned his Russian counterpart of a “united, swift and severe” response if Russia did take that step.
At a press conference after the meeting, Mr Lavrov accused Nato of working against Russia. He reiterated Moscow’s position that it has “never threatened the Ukrainian people” and has no plans to attack Ukraine.
The US will present Russia with a written response to its security red lines “next week”, he added. Another meeting is expected to follow when Russia has examined it.