ROIA, Portugal (AP) -- NATO's secretary-general sounded the alarm Thursday over the build-up of Russian military forces from the Baltic Sea to the Mediterranean and called on the U.S.-led alliance to come up with a response.

Jens Stoltenberg said the Russians have concentrated military forces in Kaliningrad, the Black Sea and the eastern Mediterranean, where they are assisting beleaguered Syrian President Bashar al-Assad.

Speaking at a news conference during NATO war games, Stoltenberg experts believe the build-up of Russian military might could lead to Moscow's ability to limit the access of the United States and its allies to certain regions.

"We have to be sure that we are able to overcome these capabilities, so we can reinforce, so we can move and we can deploy forces if needed," Stoltenberg said.

The NATO chief said the challenge of Russia's new capabilities in the field of what defense specialists call "anti-access/area denial" has become "the question on our agenda."

On Wednesday, leaders and representatives of nine Eastern European NATO member nations meeting in the Romanian capital of Bucharest called for an increased alliance presence in Europe in response to Russia's annexation of Crimea from Ukraine, and threats from the Islamic State group.

Until now, NATO has been cool to such requests, citing an agreement it concluded with Russian in 1997, when relations with Moscow were friendlier. Under the accord, NATO said it would refrain from any "additional permanent stationing of substantial combat forces." Though the alliance opened small liaison offices in capitals of its Eastern European members this year and has rotated military units in and out of countries that feel most at risk from Russia, it appears to have carefully avoided anything that might be construed by the Kremlin as the stationing of permanent reinforcements.

Stoltenberg's comments Thursday hinted that NATO and its member nations might be rethinking their approach. "The important thing is that we have military presence," the secretary-general said.