US Vice President Joe Biden said on Saturday that the United States and Turkey were prepared for a military solution against Islamic State in Syria if a political settlement was not possible, amid uncertainty over scheduled peace talks.

The latest round of Syria peace talks are planned to begin on Monday in Geneva but were at risk of being delayed partly because of a dispute over who will comprise the opposition delegation.

"We do know it would better if we can reach a political solution but we are prepared ..., if that's not possible, to have a military solution to this operation in taking out Daesh," Biden said at a news conference after a meeting with Prime Minister Ahmet Davutoglu. Daesh is the pejorative Arabic acronym for Islamic State insurgents who hold parts of Syria.

A US official clarified that Biden was talking about a military solution to Islamic State, not Syria as a whole.

The Saudi-backed Syrian opposition ruled out even indirect negotiations unless Damascus took steps including a halt to Russian air strikes.

Biden said he and Davutoglu also discussed how the two NATO allies could further support Sunni Arab rebel forces fighting to oust President Bashar al-Assad.

 Disagreement over Syrian Kurdish group

 Saleh Muslim, co-chair of the Kurdish Democratic Union Party (PYD), the main Kurdish political grouping in Syria, said on Friday the Syria peace talks would fail if Syrian Kurds are not represented.

While the United States draws a distinction between PYD, whose fighters it supports, and the outlawed Kurdistan Workers Party (PKK) in Turkey, Davutoglu reiterated the Turkish position that the PYD's military wing is part of and supported by the PKK.

The PYD's military wing, the People's Protection Units (YPG) has seized swathes of Syria from Islamic State with the help of US-led strikes and declared it an autonomous administration, to Ankara's chagrin. Davutoglu said on Saturday the YPG had become an increasing threat to Turkey.

Ankara has fought a decades-long insurgency against Kurdish PKK separatists which in July reignited into a violent confrontation with Turkish security forces.

Biden strongly criticised the PKK which is designated a terrorist organisation by the United States, the European Union and Turkey.

"ISIL is not the only existential threat the PKK is equally threat and we are aware of  that," Biden said. ISIL is another acronym for Islamic State.

"The PKK has shown no desire or inclination to do that (live in peace). It is a terrorist group plain and simple. And what they continue to do is absolutely outrageous."