Russian Foreign Minister Sergey Lavrov says Russian aircraft flying into Syria are delivering military supplies and humanitarian aid — but he shed no light on what the U.S. and NATO worry is a Russian military buildup at a Syrian airfield.

Russia has backed Syrian President Bashar Assad throughout the country's 4 ½-year civil war, providing weapons and military experts to train Syria's military.

Lavrov on Thursday once again confirmed that Russia has military personnel in Syria, but said nothing about reported deployments of additional troops.

Asked about Russian planes flying to the airfield near the Syrian city of Latakia, Lavrov said they were carrying "military goods in accordance with existing contracts and humanitarian aid."