After more than seven hours of talks in Geneva with Russian Deputy Foreign Minister Sergei Ryabkov, Sherman said the United States was ready to meet again, but that Russia had not offered assurances that it will pull back troops amassed near Ukraine.

Sherman told reporters she had offered "a number of ideas where our two countries could take reciprocal action that would be in our security interest and improve strategic stability."

She declined to give full details but said the United States made proposals on missile placement and said it was "open to discussing the future of certain missile systems in Europe" along the lines of the Intermediate-Range Nuclear Forces Treaty, from which former president Donald Trump withdrew.

"We are also open to discussing ways we can set reciprocal limits on the size and scope of military exercises and to improve transparency about those exercises -- again, on a reciprocal basis," she said in a telephonic press briefing.

She said the United States was "ready to move as expeditiously as one possibly can under these circumstances" but said that discussions would take time.

She said she ruled out a Russian call for guarantees that Ukraine will not join NATO.

"We were firm, however, in pushing back on security proposals that are simply non-starters to the United States. We will not allow anyone to slam closed NATO's open-door policy."

She renewed a call on Russia to pull back its estimated 100,000 troops.

If Russia invades, "there will be significant costs and consequences, well beyond what they faced in 2014," when Moscow seized the Crimean peninsula and backed an insurgency in eastern Ukraine, she said.

Asked if Russia was ready to remove troops, Sherman said, "I don't think we know the answer to that."

"We made it very clear that it's very hard to have constructive, productive and successful diplomacy without de-escalation because the escalation obviously increases tensions and doesn't create the environment for real negotiations," she said.

She said she told her Russian counterpart to "return the troops to the barracks, or tell us what exercises are ongoing and what your purpose is."