The United States and Russia remained deadlocked after crisis talks Monday over Moscow’s desire to block any future NATO expansion to the east, but officials agreed to continue discussions on other high-stakes security issues that the Biden administration hopes can avert another invasion of Ukraine.

Deputy Secretary of State Wendy Sherman said U.S. negotiators put forward suggestions related to the scope of American military exercises and the placement of U.S. missiles in Europe, cautioning that the bilateral discussion in Geneva, the first in a series of talks this week on Russia’s military buildup around Ukraine, was only the start of a potentially lengthy process.

“We were firm, however, in pushing back on security proposals that are simply nonstarters for the United States,” she told reporters after the seven-hour meeting. “We will not allow anyone to slam closed NATO’s open-door policy, which has always been central to the NATO alliance.”

The talks, along with parallel discussions with European officials scheduled for Wednesday and Thursday, represent a crucial test of the Biden administration’s attempt to prove that collaboration among global democracies can prevail over authoritarianism and the defiance of international norms.

Washington and Kyiv have accused Russia, which annexed Ukraine’s Crimean Peninsula in 2014, of concentrating more than 100,000 troops around Ukraine in an apparent threat of a multipronged attack. Russia says the movements are innocuous military maneuvers, but U.S. intelligence has found that Moscow is planning an offensive that could include as many as 175,000 force

Whether the talks can head off further conflict in Ukraine will probably come down to Russian President Vladimir Putin’s willingness to accept alternate security concessions from the West in lieu of the guarantees he has sought on halting NATO’s eastward growth.