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" (ISIS) terrorist group, while hailing Ankara's cooperation in the fight against jihadists, AFP reports.

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 ISIS 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 ISIS.