Russian forces are working to expand the tarmac of a major airport in Syria’s coastal province of Latakia, a stronghold of President Bashar Assad and his minority sect, a prominent Syrian monitoring group said Sunday.

The report comes amid rising concern among U.S. officials of increased Russian military activity in Syria. President Obama cast the buildup as an effort to prop up the embattled leader, warning Moscow against doubling down on Assad.

Russia, a longtime backer of Syria’s government, denies it’s trying to bolster Assad and instead says its increased military activity is part of the international effort to defeat the Islamic State group, the militant group that has wreaked havoc in both Syria and Iraq. […]