WASHINGTON — Russia has sent some of its most modern battle tanks to a new air base in Syria in what American officials said Monday was part of an escalating buildup that could give Moscow its most significant military foothold in the Middle East in decades.

Pentagon officials said that the Russian weapons and equipment that had arrived suggested that the Kremlin’s plan is to turn the airfield south of Latakia in western Syria into a major hub that could be used to bring in military supplies for the government of President Bashar al-Assad. It might also serve as a staging area for airstrikes in support of Syrian government forces.

“We have seen movement of people and things that would suggest the air base south of Latakia could be used as a forward air operating base,” Capt. Jeff Davis, a Pentagon spokesman, said Monday.

American military specialists analyzing satellite photographs and other information said Russia had about half a dozen T-90 tanks, 15 howitzers, 35 armored personnel carriers, 200 marines and housing for as many as 1,500 personnel at the airfield near the Assad family’s ancestral home. And more is on the way as Russia appears to be trying to increase its influence in Syria amid the civil strife there, the officials said.