President Vladimir Putin threatened a military response to counter NATO expansion toward Russia’s borders, but said he hoped for a diplomatic solution to rising tensions as the U.S. said it was ready to discuss his security demands.

The U.S. and its allies “must understand that we have nowhere to retreat further” and Russia can’t allow them to deploy missiles in Ukraine that would be a few minutes’ strike distance from Moscow, Putin told senior officers today at the Russian Defense Ministry. Russia will “take adequate military-technical response measures and react harshly to unfriendly steps,” he said.

Putin lashed out after Russia demanded last week that the North Atlantic Treaty Organization withdraw forces to the positions they held in 1997, before the alliance expanded to include former Soviet satellite states in eastern Europe, as part of proposed new security treaties.

The U.S. is ready to start talks on those proposals next month, though some parts of them are “unacceptable,” Assistant Secretary of State Karen Donfried told reporters today. NATO Secretary General Jens Stoltenberg said the alliance wants “meaningful dialogue” with Moscow and he plans to call a NATO-Russia Council meeting as soon as possible in the New Year.

The U.S. and Europe accuse Russia of a massive build-up of troops near Ukraine in preparation for a possible invasion as early as next month, something Russia denies. The U.S. and its allies are working on plans to impose painful new sanctions on Russia if it invades Ukraine. The West has also promised to increase arms supplies to Kyiv in the event of a Russian incursion, though NATO hasn’t announced plans for any missile deployments in Ukraine, which isn’t a member of the alliance.

“A dialogue with Russia needs to be based on the core principles of European security and to address NATO’s concerns about Russia’s actions,” Stoltenberg told reporters in Brussels. “And it needs to take place in consultation with NATO’s European partners, including with Ukraine.”

NATO says it’s a defensive body and its deployments in member states have come in response to threats from Russia. While the alliance has said the door is open to Ukraine for eventual membership, there’s little prospect of that happening any time soon.

Despite the threat of a military response, Putin said Russia’s proposed security treaties aren’t an ultimatum. “Armed conflict and bloodshed are absolutely not our choice,” he said. “We want to resolve the question by political and diplomatic means.”

But if U.S. and NATO missiles appear in Ukraine “the flight time to Moscow will be reduced to 7-10 minutes, and if hypersonic weapons are deployed – to within 5 minutes,” Putin said. “This is a serious challenge for us, a challenge for our security.”

Russia sees potential risks to its territory and Crimea if the U.S. put hypersonic missiles in Ukraine then allowed “extremists” there to carry out attacks, Putin said. “Do they think we don’t see these threats?” he said.

Russia is hoping for “constructive and substantive with visible results over a certain time period” to its security proposals, Putin said, warning that there’s also a risk that Washington will try to bog down negotiations.

Moscow can’t trust security guarantees offered by the U.S. because Washington easily walks away from its commitments, the president said. Still, “clearly stated” written guarantees are better than oral ones, he said.