Top U.S. defense officials say Syrian rebel forces are now preparing to go after the Islamic State's de facto headquarters of Raqqa after retaking the key eastern town of Shaddadeh.

The chairman of the Joint Chiefs of Staff is telling a House appropriations subcommittee that the rebels funded by the U.S. got off to a slow start. But Gen. Joseph Dunford says they're "going down to now isolate" Raqqa and have successfully cut the insurgent group's communication between Iraq and Syria.

Defense Secretary Ash Carter says the largely Kurdish Syrian Democratic forces circled Shaddadeh and want to sever the northern artery between Raqqa and Mosul in Iraq.

The rebel gains against Islamic State come as the group continues to lose ground in recent battles with the Syrian regime.