US Vice-President Joe Biden has said the US and Turkey are prepared to seek a military solution against so-called Islamic State (IS) if a political agreement in Syria proves impossible.
Mr Biden said a political deal "would be better" but "if that's not possible" the US would pursue alternatives.
US officials said Mr Biden's remarks on a deal referred to planned peace talks between Syrian officials and rebels.
They said his comment on possible military action applied only to IS.
The US vice-president was speaking after meeting Turkish PM Ahmet Davutoglu in Istanbul.
Commenting on the chance of achieving a political settlement in Syria, Mr Biden said: "We are neither optimistic or pessimistic. We are determined."
Referring to possible military action, he said this would entail "taking out Daesh", using the Arabic acronym for IS.
Mr Biden said he and Mr Davutoglu had discussed how their countries could further support Sunni Arab rebels opposing Syrian President Bashar al-Assad.
In other comments, Mr Biden said Washington recognised that the Kurdistan Workers Party (PKK), which is banned in Turkey, was as much of a threat to Ankara as IS.
"It is a terror group plain and simple and what they continue to do is absolutely outrageous," he said.
The PKK, which is listed as a terrorist group by Washington and the EU, has been accused of carrying out a number of deadly attacks on Turkish security forces since a truce collapsed in July.
The group, which has been fighting Turkey for an autonomous homeland for the Kurds for decades, has said the truce had "no meaning" after it faced assaults from Turkish forces.
Mr Biden, who also held talks with President Recep Tayyip Erdogan during his visit, on Friday strongly criticised restrictions on freedom of expression in Turkey.
He said that intimidating journalists, shutting down social media and accusing government critics of treason set a poor example to the region.