The United States and NATO rejected key Russian security demands for easing tensions over Ukraine but left open Wednesday the possibility of future talks with Moscow on arms control, missile deployments and ways to prevent military incidents between Russia and the West.

The decisions came at a meeting of the NATO-Russia Council, the first of its kind in over two years. That Russias delegation did not walk out of the talks and remained open to the prospect of future discussions after having its main positions rebuffed were seen as positive notes in a week of high-level meetings aimed at staving off a feared Russian invasion of Ukraine.

Russian President Vladimir Putin wants NATO to withdraw its troops and military equipment from countries that border Russia, which include Ukraine but also NATO allies like Estonia, Latvia and Lithuania. Putin also asked for the 30-nation military alliance to agree not to admit any more members.

Speaking after the meeting at NATO headquarters in Brussels, U.S. Deputy Secretary of State Wendy Sherman reaffirmed that some of Putins security demands are simply non-starters.

We will not slam the door shut on NATOs open-door policy, she told reporters after almost four hours of talks. We are not going to agree that NATO cannot expand any further.

The meeting was called as an estimated 100,000 combat-ready Russian troops, tanks and heavy military equipment are massed near Ukraines eastern border. The buildup has caused deep concerns in Kyiv and the West that Moscow is preparing for an invasion.

Russia denies that it has fresh plans to attack its neighbor and in turn accuses the West of threatening its security by positioning military personnel and equipment in central and eastern Europe.

While noting that escalation does not create optimum conditions for diplomacy, to say the least, Sherman also expressed optimism following the Brussels meeting given that Moscow did not dismiss the idea of further talks.

NATO Secretary-General Jens Stoltenberg, who chaired the meeting, said NATO nations and Russian envoys both expressed the need to resume dialogue and to explore a schedule of future meetings.

Stoltenberg said NATO is keen to discuss ways to prevent dangerous military incidents or accidents involving Russia and the Western allies, reducing space and cyber threats, as well as setting limits on missile deployments and other arms control initiatives.

But Stoltenberg said any talks about Ukraine wouldnt be easy. Russia annexed the Crimean Peninsula from Ukraine and backed a separatist insurgency in Ukraines eastern industrial heartland in 2014. In the years since, the fighting has killed more than 14,000 people.

There are significant differences between NATO allies and Russia on this issue of Ukraines potential NATO membership, Stoltenberg told reporters after what he said was a very serious and direct exchange with Russian Deputy Foreign Minister Alexander Grushko and Deputy Defence Minister Alexander Fomin.

Stoltenberg underlined that Ukraine has the right to decide its future security arrangements and that NATO would continue to leave its door open to new members.

No one else has anything to say, and of course, Russia does not have a veto, he said.

Grushko, who described Wednesdays talks as serious, deep and substantive, countered by saying that the freedom to choose ways of ensuring ones security mustnt be implemented in a way that infringes on legitimate security interests of others.

He did not rule out more discussions with the Western allies but scoffed at NATOs assurances that it doesnt threaten Russia and warned that the alliances attempts to ensure its security by deterring Russia were doomed to fail.

If NATO opts for the policy of deterrence, we will respond with a policy of counter-deterrence, Grushko said. If it turns to intimidation, we will respond with counter-intimidation. If it looks for vulnerabilities in Russias defence system, we will look for NATOs vulnerabilities. Its not our choice, but we dont have other options if we dont overturn this current very dangerous course of events.

The NATO-Russia Council was set up two decades ago, but full meetings paused when Russia annexed the Crimean Peninsula nearly eight years ago. It has met only sporadically since.

Moscows draft agreement with NATO countries and the offer of a treaty between Russia and the United States would require NATO to halt all membership plans, not just with Ukraine, and scale down its presence in countries close to Russias borders.

Endorsing such an agreement would mean NATO abandoning a key tenet of its founding treaty, which holds the alliance can invite in any willing European country that can contribute to security in the North Atlantic area and fulfill the obligations of membership.

The Russian draft also proposed mutual limits on war games and confidence-building measures to prevent accidents involving warships and aircraft. Grushko said Russia would be willing to continue discussions on those issues and arms control steps, such as the non-deployment of intermediate-range missiles in Europe.

In the United States on Wednesday, Senate Democrats released their White House-backed proposal for legislation that would ratchet up sanctions on Russia if it sends troops into Ukraine. The measures would target Putin, his top civilian and military leaders, and leading Russian financial institutions.

Vladimir Isachenkov in Moscow, Ellen Knickmeyer in Washington, and Jari Tanner in Tallinn, Estonia, contributed to this report.