WASHINGTON — The United States and Russia continue to be at odds with each other over a military verification treaty, to the point where no flights have been conducted in 2018.

The latest issue with the Open Skies Treaty came to light earlier this week, with Russian officials saying the U.S. has refused to clear its planes for overflight of U.S. territory. Under the treaty, 34 countries, including both the U.S. and Russia, agree to allow unarmed surveillance flights over their territory to provide information gathering about military forces.

“In breach of the Open Skies Treaty provisions, the head of the U.S. delegation refused to sign the final document, without giving any explanations or reasons, and citing direct instructions from Washington,” said Sergei Ryzhkov, the chief of Russia’s Nuclear Risk Reduction Center, according to the Tass news agency.

“We insist that the U.S. side return to the Open Skies Treaty framework and demand that the current situation be explained with reference to the treaty’s provisions,” Ryzhkov said.

This comes on the heels of Russian news reports over the summer claiming the U.S. had dropped out entirely of the agreement, something a U.S. State Department official denied was the case.

However, that official acknowledged there have been no Open Skies flights conducted in 2018 thanks to ongoing disagreements between the two nations.

“To the best recollection of our experts, we have not denied any Russian flights that were conducted in accordance with the Treaty,” the official said.

Speaking at the Defense Writers Group on Sept. 7, Andrea Thompson, U.S. undersecretary of state for arms control and international security, said talks are at an “impasse” between the two countries.

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“It’s at an impasse, but we’re having discussions. There are some things that Russia needs to do to get back into compliance with that,” Thompson said at the time. “We’re having those discussions, they’re ongoing. So that’s the most important part, that the dialogue is occurring and to get them back in compliance. And then we’ll move forward.”

In February 2016, Russia announced plans to add a new digital electro-optical sensor to its Tupolev Tu-154 aircraft used for Open Skies flights. Pentagon officials and lawmakers alike raised the alarm that the new sensors would give Russia an informational edge over what can be gathered by the equipment used by the U.S.

Things got worse in early 2018, with both nations slapping each other with restrictions. Russian officials blocked the U.S. from using these designated Russian military bases as overnight hubs for its flights, while the U.S. closed two American bases from Russian crews for the same use.

The goal of the treaty, proponents say, is to provide open information that can be used to confirm adherence to arms-control treaties. But the agreement has been a target of Congress in the past, who have argued the treaty gives Russia a strategic edge.

Sen. Tom Cotton, R-Ark., reacted to Russia’s claim on Wednesday, tweeting: “It’s rich for the Russians to protest the U.S.’s refusal to certify one of their planes for the Open Skies Treaty when they routinely restrict surveillance flights over Kaliningrad. The Open Skies Treaty is out of date and favors Russia, and the best way forward is to leave it.”

Such comments are troubling to supporters of the treaty, such as Kingston Reif of the Arms Control Association.

“For several years, GOP hawks in Congress, with the support of some treaty skeptics in the Pentagon, have sought to cripple U.S. implementation of the treaty. Now it appears these voices are making U.S. policy and accelerating their efforts to kill the treaty,” Reif said. “The treaty mandates information sharing about military forces that increases transparency among members, thereby contributing to stability and improving each participating state’s national security.”

The 2019 National Defense Authorization Act, signed by U.S. President Donald Trump in August, requires the executive branch to fulfill certain reporting and certification requirements before money could be released for the Open Skies program. In essence, that language restricts updates for the systems used by the U.S. to track Russian nuclear sites.

The language is the result of congressional fights over whether or not the U.S. Defense Department should use funds to upgrade the OC-135B, an aging airframe that has struggled with maintenance rates and, in at least one memorable case, had to make an emergency landing in Russia during an overflight mission.