Defense Secretary Jim Mattis warned Russia Thursday that Washington will not accept Moscow's continued violation of the Intermediate-Range Nuclear Forces Treaty with its covert development of land-based cruise missiles that could threaten Europe.

“Make no mistake: The current situation with Russia in blatant violation of this treaty is untenable,” Mattis said at the conclusion of a meeting of NATO defense ministers in Brussels.

“We discussed this situation at length during this ministerial meeting among trusted allies,” Mattis said. “The United States is upholding its arms control obligations. Russia is not, and it is time now for Russia to return to compliance."

Mattis is the second high-ranking U.S. official this week to signal that Washington is losing patience with Moscow over its alleged violation of the Cold War era treaty signed by former President Ronald Reagan and Russian leader Mikhail Gorbachev in 1987, which bans ground-launched ballistic and cruise missiles with ranges of 500 to 5,500 kilometers.

“We have been trying to send a message to Russia for several years that we know they are violating the treaty. We have shown Russia the evidence that we have, that they are violating the treaty,” U.S. Ambassador to NATO Kay Bailey Hutchison said on Tuesday.

“They are building a medium-range ballistic missile in violation of the INF. That is a fact which we have proven,” she told reporters in Brussels.

Hutchison briefly caused a stir when she seemed to threaten military action to take out the weapons once they are deployed, but then walked back her comments in a tweet, insisting she was not talking about a pre-emptive strike.

Mattis was more measured, but also indicated the U.S. would not let the deployment go unanswered.

“Russia must return to compliance with the INF Treaty, or the U.S. will need to respond to its cavalier disregard of the treaty's specific limits,” he said.

Mattis told reporters that the U.S. was reviewing options in its “diplomacy and defense posture,” but would act in concert with its NATO allies.

One option would be to scrap the three-decades-old treaty and for the U.S. to develop and field its own land-based cruise missile.

The 2018 National Defense Authorization Act directs Mattis to establish a program to develop a system that would match the Russia ground-launched cruise missile system and authorized $58 million to fund development of active defenses to counter intermediate-range ground-launched missile systems.

The treaty permits research, but not actual development, so the U.S. can carefully keep within the limits of the INF Treaty.

“The United States does not want to withdraw from the treaty. We certainly don’t intend to violate the treaty,” said Hutchison. “But if Russia continues to say they are not violating when the evidence is clear that they are, then diplomacy needs to be strengthened, and we need to look for other ways to bring Russia to the table on this issue.”

Mattis stressed for now the U.S. would prefer that the Russians abandon their provocative program rather than engage in an arms race.

“We are trying to bring them still back into compliance. And now is the time; it's gone on long enough,” Mattis said. Ultimately it will be up to President Trump to decide on a course of action, and Mattis said he’ll take to heart the input he received from other NATO defense ministers.

“I prefer not to say how we'll respond. We have, again, a host of ways that we can respond,” he said. “We will respond as we think is appropriate.”