Officials say U.S. will move ships into Baltic Sea and take over North Atlantic Treaty Organization air-policing duties

A military vessel is seen during Russia’s Zapad exercises in 2013.

VILNIUS, Lithuania—The U.S. will bolster its ability to observe Russia’s military in the Baltic Sea region ahead of a major exercise by Moscow, American defense officials said.

The officials said the Pentagon would enhance its surveillance ability, including by moving ships into the Baltic Sea and taking over North Atlantic Treaty Organization air-policing duties while the Russian exercises occur in September. The U.S. could temporarily deploy a Patriot missile-defense battery in the region, officials said.

Defense officials said the Russian drill, known as Zapad, will showcase some of Russia’s newest and most advanced weaponry, and will be a chance for the U.S. to gather fresh intelligence.

They said Russia could use the exercise to upgrade its military equipment in the area, leaving behind some of the advanced systems in the region after the exercises conclude.

U.S. Defense Secretary Jim Mattis, appearing here on Wednesday with Lithuanian President Dalia Grybauskaitė, declined to say what specific systems the U.S. could deploy to the region to support deployed NATO force.

“We are here in a purely defensive stance,” Mr. Mattis said. “Everyone knows this is not an offensive capability. Anyone who says otherwise, I would just say I have too much respect for the Russian army to think they believe there is any offensive capability.”

Mr. Mattis is meeting his counterparts from Lithuania, Latvia and Estonia as part of a trip designed to showcase how the U.S. and NATO are shoring up allied defenses against Russia and its troop buildup.

The visit to Russia’s border also comes as questions swirl in Washington over whether the White House firing of FBI Director James Comey could hamper the investigation into the Trump campaign’s ties to Russia.

Ahead of the Russian exercise, which will take place in Belarus, the U.S. is considering moving a Patriot missile defense battery into the Baltic States region as part of their own allied drills in July, defense officials said. The U.S. could keep the battery in the region longer, for the duration of the Zapad exercise, but officials have said they do not intend to place a Patriot unit in the region permanently.

U.S. and NATO officials have eyed Russia’s exercises warily, worried that it is a potential time for a miscalculation or crisis. Both sides will be deploying significant military resource in the area and Sweden, a close partner of NATO, will conduct its own multinational exercise with the U.S. and other allies.

Allied officials have also been worried about Russia’s move to upgrade its weaponry in the region, including the deployment of nuclear-capable Iskandar missile systems in Kaliningrad, a Russian exclave bordering Poland, in a move Western officials have criticized.

“Any kind of buildup like that is simply destabilizing,” Mr. Mattis.

The U.S. and NATO allies this year deployed roughly 4,000 troops to Poland and the Baltic States. Russian officials have repeatedly criticized the NATO buildup as misguided and a move that has eroded regional security.

Baltic State officials, including the Estonian defense minister, have said they expect the Russians to leave troops behind in Belarus after the exercise, as a warning or threat to NATO nations.

The stepped-up observation of the exercise is particularly important because Moscow has been secretive about its military exercises and this exercise is taking place near NATO territory, according to allied officials.

While Russian officials have discussed the exercise with alliance officials in meetings of the NATO-Russia Council, they have not said they would allow observers. The U.S. and NATO say they routinely allow Russia to observe their exercises, as required by international agreements.

U.S. officials estimate between 70,000 and 100,000 troops could take part in Zapad. Moscow has characterized the drills as far smaller, saying they will involve only a few thousand troops, below the number required to allow NATO to send a formal observation team.