NATO's secretary-general has sounded the alarm 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 Thursday that the Russians have concentrated military forces in Kaliningrad, the Black Sea and the eastern Mediterranean, where they are assisting Syrian President Bashar al-Assad.

Stoltenberg, speaking at a news conference during NATO war games in Portugal, said some experts believe the force concentrations could lead to Moscow's ability to limit NATO's access 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," said Stoltenberg.