The United States and its European allies appeared no closer to resolving a crisis with Russia over a possible renewed war in Ukraine, as Western officials flatly rejected Moscow’s call to pledge there would be no further eastward expansion of NATO, while a top Russian negotiator said diplomacy already had reached a “dead end.”

The impasse came after three rounds of high-stakes talks this week in Geneva, Brussels and Vienna, where the United States hoped to placate Russia by proposing reciprocal restraints on missile placements and military exercises in Europe. The Russian delegation expressed interest in those areas but still accused Washington and its allies of stonewalling on the Kremlin’s core demands.

This week’s flurry of meetings was seen as a critical bid to defuse a crisis amid fears that Russia — which has amassed more than 100,000 forces near its western border — could once again invade Ukraine, the neighboring nation where Russia has been fueling a separatist conflict for nearly eight years.

Massing troops near Ukraine as leverage, Putin has demanded sweeping security guarantees from the United States and NATO, including a halt to any NATO expansion, a rollback of the alliance’s infrastructure and weaponry to its 1997 boundaries — which would remove any military presence in Poland and the Baltic States, among other countries — and a ban on offensive strike weapons near Russia’s borders.

The Russian stance would fundamentally reshape European security and reflects Putin’s attempt to rework the fallout of the collapse of the Soviet Union, which left Russia weakened as NATO expanded. U.S. officials have called his demands regarding NATO non-starters.