FORT LEONARD WOOD: The United States is continuing to talk with ally Turkey on the role it will play in the operation to seize the city of Raqqa, Islamic State's main stronghold in Syria, US Secretary of Defense Ash Carter said on Wednesday.

Carter's comments, made during a news conference, come days after Turkey said it wants the Raqqa operation to start after Mosul and Euphrates Shield operations have been completed.

"We'll continue to talk with Turkey about its role in the eventual seizure of Raqqa, but we're proceeding now with the operation according to our plan," Carter said.

Carter said last week that Washington expected the Raqqa operation to overlap with the battle to retake Mosul from Islamic State.

"We intend to go there soon with the force that is capable of doing that and enveloping the city of Raqqa... the final seizure of Raqqa, we continue to talk to Turkey about that and a possible role for Turkey in that further down the road," Carter said.

Kurdish YPG militia fighters will be included as a part of the force to isolate the Islamic State-held Syrian city, the top U.S. military commander in Iraq has said.

Arab forces are expected to be the ones to take the city itself, US officials have previously said.

The United States regards the YPG as an ally in its fight against Islamic State, but Turkey regards it as a terrorist organization because of its links with Kurdish militants fighting a three-decade insurgency in Turkey.

Turkey has said that the Raqqa operation should be carried out by local forces and the Kurdish YPG militia should not be included.

Iraqi forces battled Islamic State fighters on the eastern edge of Mosul on Tuesday as the two-week campaign to recapture the jihadists' last main bastion in Iraq entered a new phase of urban warfare.