A U.S. Air Force reconnaissance plane was intercepted by a Russian fighter plane over the Baltic Sea in what American officials said on Sunday was an unsafe and unprofessional manner, the second such instance in a week.

Russian officials disputed the account, saying they were responding to an unidentified target approaching their border at high speed.

The incident, which happened Thursday but didn’t come to light until this weekend, could further complicate U.S.-Russia relations. As the U.S. builds up its assets in the Baltic Sea, Russia has taken a more aggressive posture in response.

The disconnect over each side’s intentions has resulted in an increase of incidents between the American and Russian militaries in the region, according to U.S. officials. Russia sees the U.S. operating in its own backyard and doesn’t like it. That has resulted in a “game of chicken,” said Eugene Rumer, who directs the Russia and Eurasia Program at the Carnegie Endowment for International Peace, a think tank in Washington.

“They don’t want to take on the U.S. in an outright military confrontation, but the way I read this body language of theirs, they are not just going to stand by and watch it, they are going to make it difficult,” Mr. Rumer said. “They will use the entire toolkit they have, and harassment is part of the arsenal.”