Moscow and Washington aren’t budging. Officials from both governments are meeting amid a Russian troop buildup near Ukraine. The United States has rejected Russia’s demand to halt expansion by its allies—while the Kremlin says it may stop negotiating altogether.

Russia wants the Ukraine and other ex-Soviet nations to be restricted from joining the alliance called the North Atlantic Treaty Organization (NATO). At Monday’s talks in Switzerland, Russia also demanded a rollback of the military alliance’s presence in Eastern Europe. But the United States firmly rejected those demands.

The U.S. envoy to NATO ruled out any giving in on the alliance’s eastward expansion. “We will not allow anyone to slam NATO’s open-door policy shut,” says U.S. Ambassador Julianne Smith. The policy states that no non-member country has the power to keep NATO from admitting new members.

Kremlin spokesman Dmitry Peskov describes the Geneva talks as “open, comprehensive, and direct.” But he emphasizes that Moscow wants quick results.

Peskov says upcoming Russia-NATO talks in Brussels and Vienna would show whether further discussions are worthwhile.

“It will become clear in what direction and how to proceed and if it makes sense,” he says. “We absolutely wouldn’t accept dragging this process out endlessly.”

The United States estimates that Russia has gathered about 100,000 troops near Ukraine. That buildup stokes fears of an invasion. Moscow says it has no plans to attack. Officials there reject Washington’s demand to pull back forces. Russia claims it has the right to deploy its troops wherever necessary.

President Vladimir Putin has warned that Moscow would take unspecified “military-technical measures” if the United States and its allies don’t meet its demands.

Meanwhile, the United States warns that Russia will face unprecedented sanctions if it attacks Ukraine.

White House press secretary Jen Psaki says it’s “too early to tell whether the Russians are serious about the path to diplomacy or not.” She cautions that they may use the talks as an “[excuse] to claim that diplomacy couldn’t possibly work”—and then invade.

In 2014, Russia annexed Ukraine’s Crimean Peninsula after the removal of its Moscow-friendly leader. (Read Crimea after Russian Takeover.) Despite a 2015 peace deal brokered by France and Germany, frequent skirmishes have continued.

Ukrainian President Volodymyr Zelenskyy met Tuesday with French and German officials. He’s considering another meeting of the leaders of Russia, Ukraine, France, and Germany. Zelenskyy says he wants “substantive talks on ending the conflict.”

U.S. Deputy Secretary of State Wendy Sherman has briefed the North Atlantic Council on the Geneva talks. She tweeted, “The United States is committed to working in lockstep with our allies and partners.” Sherman called the goal “to urge de-escalation and respond to the security crisis caused by Russia.”