WASHINGTON— President Barack Obama is seeking to bolster U.S. efforts to train and arm select members of the Syrian opposition, a move that comes amid increased U.S. concern that the conflicts in Syria and Iraq are becoming an intertwined fight against the same Sunni extremist group.

Obama sent Congress a $500 million request Thursday for a Pentagon-run program that would significantly expand previous covert efforts to arm rebels fighting both the Sunni extremists and forces loyal to Syrian President Bashar Assad. If approved by lawmakers, the program would in effect open a second front in the fight against militants spilling over Syria’s border and threatening to overwhelm neighboring Iraq.