US Secretary of State Hillary Clinton warned Saturday that Syria must not become a haven for the Kurdistan Workers' Party (PKK) rebels battling Turkey.

"We share Turkey's determination that Syria must not become a haven for PKK terorrists whether now or after the departure of the Assad regime," Clinton told a joint news conference in Istanbul with his Turkish counterpart Ahmet Davutoglu.

The Democratic Union Party (PYD), the Syrian ally of the PKK, has allegedly seized control of several towns along Turkey's border with Syria, alarming Ankara, which promptly increased defences on the border.

At the press conference, Davutoglu warned against a "power vacuum" in conflict-torn Syria being exploited by the PKK and said, "We need to take joint efforts to prevent a power vacuum from being formed."