BEIRUT- The main Syrian Kurdish political party, the PYD, on Wednesday welcomed a longer-term role for U.S. forces in Syria after the defeat of Islamic State, saying the Americans should continue to play a role until a political solution to the Syrian crisis.

Syrian Kurdish groups have been the main partner on the ground for a U.S.-led coalition fighting against Islamic State in northern and eastern Syria.

The Kurdish fighters, with Arab allies, U.S. advisers and air support from the U.S.-led coalition, have driven Islamic State from swathes of territory including the de-facto Syrian capital of the group's self-proclaimed caliphate, Raqqa.

The PYD has established autonomous rule in territory its allied militia control, and says it wants a federal model for Syria. Its advances anger neighbour Turkey, which considers it an ally of the banned Kurdistan Workers' Party that has fought a decades-long insurgency against the Turkish state.

On Monday, U.S. Defense Secretary Jim Mattis described a longer-term role for U.S. troops long after Islamic State militants lose control of all the territory they hold.

In a written message to Reuters, the PYD's co-president, Shahoz Hasan, agreed that this would be beneficial.

"Without achieving a political solution to the Syrian crisis, and with the continuation of the Turkish and Iranian intervention in Syria, and with the continued presence of al Qaeda groups in Syria, the continued operation of the coalition is better," Hasan said.

While all sides in Syria were battling against Islamic State, the U.S.-backed, Kurdish-led forces mostly avoided direct confrontation with the Syrian government, backed by Iran and Russia. But as Islamic State has been driven from nearly all its territory in recent months, the government and its Iranian allies have increasingly spoken of taking back areas held by the Kurdish-led militias.

Syria's main Kurdish groups hope for a new phase of negotiations to shore up their autonomy in northern Syria.

Mattis said the U.S. military's longer-term objective would be to prevent the return of an "ISIS 2.0".

But he also suggested that U.S. forces aimed to help set the conditions for a diplomatic solution in Syria, saying "we're not just going to walk away right now before the Geneva process has traction", a reference to U.N.-backed peace talks.

In response to the Mattis statement, the Syrian government said on Tuesday that Washington was presenting a new excuse to keep its forces in Syria by linking this presence to a political settlement, having previously said its goal was to fight IS.

A foreign ministry statement affirmed the government's position that the presence of U.S. and other forces in Syria without government approval was an act of aggression.

A top adviser to Iran's supreme leader said last week that he expected the Syrian army to recapture eastern Syria soon. An adviser to Syrian President Bashar al-Assad also said last week that Damascus would not give up on Raqqa.

(Reporting by Tom Perry; Editing by Peter Graff) ((thomas.perry@thomsonreuters.com))