The U.S. criticized Russia Thursday for an incident this week during which a Russian Su-24 military jet came within 30 feet of a U.S. naval destroyer in international waters in the Baltic Sea. Tuesday’s encounter followed a similar incident Monday involving a pair of Russian jets in a move U.S. military officials called “aggressive,” Agence-France Presse reported. 

“We have deep concerns about the unsafe and unprofessional Russian flight maneuvers,” the U.S. military’s European Command said in a statement Thursday. “These actions have the potential to unnecessarily escalate tensions between countries and could result in a miscalculation.”

Russian authorities in turn said they were surprised by the U.S. reaction, insisting the planes were carrying out normal maneuvers. The Russian jets were conducting test flights and moved out of the U.S. ship’s airspace in a timely manner, Defense Ministry spokesman Igor Konashenkov said in a statement posted to the ministry’s Facebook page.

“Frankly speaking, we don’t understand such a sore reaction from our U.S. colleagues,” Konashenkov said.

Vladimir Putin April 2016 Russian President Vladimir Putin attends a meeting with the French Senate president at the Kremlin, April 5, 2016. Photo: IVAN SEKRETAREV/AFP/Getty Images

Relations between the U.S. and Russia have grown increasingly strained following Russian annexation of the Crimean peninsula in 2014. An ongoing civil war in Syria has alternately brought the two countries together and driven them apart as they have been forced to work together while disagreeing on the future of Syrian President Bashar Assad.

Russia has been steadily increasing its presence in the Arctic Circle and the Baltic Sea in the past several years. The nation completed a military base in the Arctic Circle in October, according to the Defense Ministry. The base measures 150,000 square-feet and can house 150 soldiers for up to 18 months without the need to return for supplies. Meanwhile, Moscow has filed land claims over vast swathes of the Arctic Circle, including the North Pole, with the U.N.