The United States' Central Intelligence Agency (CIA) chief on Tuesday met with the Turkish prime minister on discussions over the Syrian crisis.

CIA chief David Petraeus, who paid a two-day surprise visit to Ankara, also held talks with Prime Minister Recep Tayyip Erdogan on their cooperation in fighting the outlawed Kurdish Workers' Party (PKK).

Turkey is scheduled to host the second so-called "Friends of Syria" meeting on April 2, which claims to work towards a political solution to the year-long violence in Syria and a humanitarian aid corridor to help the Syrian civilians.

Erdogan and Petraeus also discussed the activities of the PKK in northern Iraq, where Turkish fighter jets have frequently conducted airstrikes since last summer.

The United States and Turkey have been closely cooperating in intelligence sharing on PKK activities and its camps in northern Iraq.

Listed as a terrorist group by Turkey, the United States and the European Union, the PKK took up arms in 1984 trying to create an ethnic homeland in southeastern Turkey. More than 40,000 people have been killed in conflicts involving the PKK over the past over two decades.