Istanbul (AFP) - US Vice President Joe Biden on Saturday condemned the Kurdistan Workers Party (PKK) as a "terror group plain and simple" and a threat to Turkey like the Islamic State group, while hailing Ankara's cooperation in the fight against jihadists.

After talks with Turkish Prime Minister Ahmet Davutoglu in Istanbul, Biden pleased his hosts with a ringing denunciation of the "outrageous" attacks by the PKK, which the Turkish military is seeking to cripple in a relentless but controversial campaign.

He praised much closer cooperation in the battle against IS between Washington and Ankara, which had been accused in the past of turning a blind eye to the jihadist extremists.

He also spoke of the possibility of a "military operation" in Syria should peace talks fail, but the White House later made clear he was only referring to current coalition military operations against IS.

The PKK, listed as a terror group by Washington and the European Union, has killed dozens of members of the Turkish security forces in attacks since a truce collapsed in July.

Turkey however has expressed concern that Washington was seeking to woo the PKK as an ally in the anti-IS fight and vehemently opposed any move to delist it as a terror group.

Ankara is also concerned about the extent of US cooperation in Syria with the People's Protection Units (YPG), which Turkey sees as a branch of the PKK.

The IS group "is not the only existential threat to the people of Turkey, the PKK is equally a threat and we are aware of that... it is a terror group plain and simple and what they continue to do is absolutely outrageous," Biden said.

"We do agree with you that in terms of their actions Daesh (IS), PKK and (Al-Qaeda's Syria affiliate) Al-Nusra... are simply terrorist groups," said Biden.

Davutoglu emphasised Ankara saw no difference between those groups and the extreme-left DHKP-C. "We were pleased to have seen in the talks our approach is shared," he said.

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He also reaffirmed Turkey's opposition to the YPG, which he accused of carrying out ethnic cleansing in northern Syria and being "part of the PKK and receiving open support from the PKK."

Biden later held over two-and-a-half hours of talks with President Recep Tayyip Erdogan but no statement was made after the discussions.

- 'Shared mission' -

The US vice president praised Turkey for taking "very important steps to improve the security of its border" from IS militants who until recently controlled much of the frontier region in Syria.

Biden said Turkey and the United States were coordinating ever more precisely on what is a "shared mission on the extermination of IS".

"We do believe our plans together have gotten more contoured and more coordinated," he said.

"We are increasingly making progress and that progress will be sped up as a result of our meeting today."

He said the two sides also discussed bolstering support for Sunni Arab groups inside Syria to cut off any remaining access that IS has to the border region.

"That is a priority for both our nations so we can prevent new fighters and equipment from reaching IS fighters," he added.

Biden said work was continuing on finding a political solution to Syria's almost five-year civil war, saying: "We are neither optimistic nor pessimistic. We are determined."

He added: "But we are prepared if that is not possible to having a military solution to this operation and taking out Daesh (IS)," he said.

A US official emphasised that Biden's comments did not herald any change in US policy in Syria, saying he was referring to the simultaneous pursuit of a political solution alongside the fight against IS.

- Controversial talks -

Biden had Friday ratcheted up tensions with a stinging attack on Turkey's current record on freedom of expression, saying it is "not the kind of example that needs to be set".

Davutoglu hit back by expressing surprise at Biden's schedule of meetings on Friday where he had met anti-government journalists and the son and wife of the jailed editor of the Cumhuriyet daily Can Dundar.

In comments in pro-government newspapers Saturday, Davutoglu said "we would have expected" Biden should also have met with victims of PKK attacks.