Senior Pentagon officials say more Arab fighters are joining Syria's U.S.-backed militia due to its success against the Islamic State group and other militants.

The chairman of the Joint Chiefs of Staff tells a Senate committee that the number of Arabs in the predominantly Kurdish Syrian Democratic Forces has doubled over the last month, from 2,500 to 5,000.

Gen. Joseph Dunford says a "snowball effect" is responsible for the growth.

He says Arab fighters are more willing to join because they see the amount of support that the U.S. is providing and the level of success that these forces are having.

Defense Secretary Ash Carter wants lawmakers to quickly provide nearly $50 million that the Pentagon has requested to support the indigenous forces in Syria.