US Defense Secretary Ashton Carter on Thursday praised the fighting of Kurdish groups in Syria that have declared a federal region in areas of the country under their control.
The Kurds "have proven to be excellent partners of ours on the ground in fighting ISIL," Carter said, using a term for the Islamic State group in testimony before the Senate Armed Services Committee.
"We are grateful for that, and we intend to continue to do that, recognizing the complexities of their regional role."
He spoke the same day Syria's Kurds declared a de facto federal region in areas under their control in the north of the conflict-riven country, a move the United States opposes.
Kurdish groups say declaring a federal region along Syria's northern border with Turkey is aimed only at formalizing a semiautonomous zone they have already established during five years of war and create a model for decentralized government across Syria.
But Washington believes any Syrian federalism should develop from United Nations-sponsored talks over a political settlement of the conflict currently underway in Geneva.
However, none of the groups representing the Kurdish minority are participating in the talks because of opposition from Turkey.
The Syrian government and the main Arab-led opposition group oppose federalism in Syria.
So does Turkey, which fears Kurds' ambitions in Syria are fuelling Kurdish separatism at home.
Kurds have played a crucial role in the US-led fight against the Islamic State group in Syria, providing the backbone of the forces that have pushed back the jihadist group in the country's northeast.
The Kurds play a dominant role in the Syrian Democratic Forces (SDS), an Arab-Syrian coalition the United States supports.
However, the Pentagon believes more Arab fighters will join the group.
"One month ago I would have said there are about 2,500 Arabs inside Syrian Democratic Forces," said Joint Chiefs of Staff chairman Gen. Joe Dunford, who spoke alongside Carter. "Today, I can tell you we have 5,000."
More coalition victories would have a "snowball effect," he added.
"More people now are willing to join us because they see the level of support that we are providing and more importantly the level of success that these forces are having."
SDS fighters aided by US special forces recently took control of Al-Shadadi, a town in northeastern Syria previously considered a strategic Islamic State group stronghold.
The SDS has a total of 10,000 to 15,000 fighters, Dunford said, adding that they have an estimated reserve of 20,000 to 30,000 men.