STOCKHOLM--Sweden Friday lodged a formal protest with the Russian ambassador here over a violation of Swedish airspace by two Russian war planes in the latest sign of rising tension between Moscow and its Western neighbors.

The Swedish foreign ministry said the planes flew in and out of Swedish airspace south of the island of Oland in the Baltic Sea Wednesday.

Foreign Minister Carl Bildt told Swedish public-service television this was the first time in his eight years in the post that his ministry had deemed it necessary to call in an ambassador to complain over a violation of Swedish airspace.

"We consider this serious," he said.

A spokesperson for the Russian embassy in Stockholm couldn't be immediately reached for a comment.

The violation occurred at the same time as Mr. Bildt was meeting with Sweden's foreign affairs and defense committees to discuss military developments in the Baltic region and it is the latest in a series of flair-ups between Russia and its neighbors.

In late August, Russian aircraft violated Finnish airspace three times in a week and in March last year Russia carried out air exercises over the Baltic Sea and war planes flew toward the Swedish border.

Tensions are also high in the Baltic States after an Estonian intelligence officer was detained by Russian authorities on suspicion of spying earlier this month.

Estonian officials say the officer, Eston Kohver, was working on the Estonian side of the border and was snatched by individuals who came from Russia. Russia, meanwhile, says Mr. Kohver was arrested while on a spying mission within its borders.

Mr. Kohver remains in Russian detention.

Unlike the Baltic states, neither Sweden nor Finland is a member of the North Atlantic Treaty Organization but they have been moving closer to the defense alliance as tensions between Russia and the West have risen over Russia's actions in Ukraine.

The two Nordic states recently signed an agreement making it easier for troops from the alliance to operate on Swedish and Finnish territory.

Write to Charles Duxbury at charles.duxbury@wsj.com and Christina Zander at christina.zander@wsj.com

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