A Syrian group says Russian forces are expanding the tarmac of a major airport in Syria's coastal province of Latakia, a stronghold of President Bashar Assad and his minority sect.

The head of the Britain-based Syrian Observatory for Human Rights, Rami Abdurrahman, says Sunday that witnesses, including officials inside the airport, describe the development as planned to ensure larger planes can land in the airport that houses a military base. The witnesses tell the Observatory no Syrian military or civilian officials are allowed near the tarmac.

Abdurrahman said Russian planes arrived in recent weeks carrying military equipment and hundreds of Russian military advisers. The Associated Press reported there were plans to build a military base there.

U.S. President Barack Obama expressed concern about increased Russian military activity in Syria.