US Secretary of State Rex Tillerson said on Thursday the fate of President Bashar al-Assad was up to the people of Syria, in the clearest indication yet of the new administration's policy in the war-torn country.
He also insisted during a visit to Turkey there was no difference between Ankara and Washington over the fight against the Islamic State group, even as his Turkish counterpart reiterated a key point of discord.
"I think the... longer term status of president Assad will be decided by the Syrian people," Tillerson told a news conference with Turkish Foreign Minister Mevlut Cavusoglu.
Under Barack Obama's administration, the US made Assad's departure a key policy goal, but new US President Donald Trump has put the accent firmly on defeating IS in Syria and Iraq.
US-backed forces are battling IS as they advance on the jihadists' Syrian stronghold of Raqa, laying the groundwork for an assault on their so-called "caliphate".
Tillerson's trip comes the day after Turkey announced the end of "Euphrates Shield", its own military offensive in northern Syria launched in August, but did not say if its troops had been withdrawn.
Ties between the Nato allies were strained under Obama, particularly over US cooperation with the Syrian Kurdish militia fighting against IS, and the issue of a US-based Turkish preacher blamed by Ankara for orchestrating the attempted coup last year.
Ankara views the Syrian Kurdish People's Protection Units (YPG) as a "terror group" linked to Kurdish separatists waging an insurgency inside Turkey since 1984, but Washington regards them as the best force fighting IS.
Turkey has suggested it wants to join any operation to capture Raqa, but without the Kurdish militia.
Tillerson hailed Turkey as a "key partner" in the fight against IS jihadists.
"There's no space between Turkey and the US and our commitment to defeat Daesh, to defeat ISIS," he added.
He said options to defeat IS "anywhere Daesh shows its face" were difficult.
"What we discussed today were options that are available to us... These are not easy decisions. There are difficult choices that have to be made," Tillerson said, without elaborating.
"In terms of the future of Raqa we look forward to the liberation of Raqa and return of its control to local citizens authorities putting it under local control for security... so that all of the Syrians who had to flee that area can return."
However, Cavusoglu said Ankara expected "better cooperation" with the Trump administration regarding the YPG.
"It is not good or realistic to work with a terror group while fighting another terror group," he said.
Numerous diplomatic efforts have failed to end the Syrian conflict, which has killed more than 320,000 people and displaced millions since it erupted in March 2011 with protests against Assad's regime.
A fifth round of UN-sponsored peace talks is taking place in Geneva but no breakthrough has been reported and they are scheduled to end on Friday.
Mark Toner, acting State Department spokesman, had said earlier this month that Washington saw Assad as "a brutal man who has led his country into this morass" who could not be "an acceptable leader to all of the Syrian people".
"That said, it's up for the Syrian people -- that means opposition, moderate opposition - working with... some representation on the part of the regime to try to forge a political transition."
Tillerson also met President Recep Tayyip Erdogan for over two hours and held talks with Prime Minister Binali Yildirim.
Another bone of contention between Ankara and Washington is Turkey's call for the extradition of Turkish cleric Fethullah Gulen.
Turkey accuses Gulen, an erstwhile Erdogan ally who lives in self-imposed in Pennsylvania, of ordering the July military coup bid, charges he strongly denies.
Yildirim's office said he and Tillerson discussed the next steps that should be taken for Gulen's return to Turkey.
The US detention of a senior Turkish state bank executive this week has also raised tensions, with Cavusoglu describing it as "completely political".
Halkbank's Mehmet Hakan Atilla is accused of helping to process millions of dollars of illegal transactions through US banks for Iran's government and other Iranian institutions.