Turkey must "do more" to tackle "Islamic State" (IS) in Syria and Iraq, said US Defense Secretary Ashton Carter on Thursday.

He called on Ankara to secure its southern borders, which has seen a flow of fighters and supplies reach militants in the two war-torn countries.

Speaking at a Pentagon news conference, Carter said: "We need them also as a neighbor to this conflict zone, as a long-time NATO ally and a responsible member of the anti-ISIL coalition, to control the border, the long border that they have with both Syria and Iraq, more than it has been controlled over the last year."

Turkey has been a hesitant partner in the battle against IS, which has seen Kurdish militia play a key role in recapturing parts of Iraq and Syria from IS militants.

Historic conflicts with Kurdish groups in southern Turkey led to concerns that Ankara could end up inadvertingly supporting a Kurdish insurgency in southern Turkey. Recently, renewed fighting has broken out in the south, which has unnerved Turkish leaders.

Turkey finally agreed to join the international air campaign against IS after months of indecision although the details are still being worked out. Its own warplanes have joined the airstrikes against the jihadist group's positions and Turkish air bases are allowing US planes to fly combat missions over their neighbors.

Most US aerial combat missions over Iraq and Syria are being flown from more distant air bases in Qatar and elsewhere in the Persian Gulf region. The US also is flying F-16s from Muwaffaq Salti air base in Jordan.

Officials have said that the F-16s could also be used to fly missions into Iraq.

There have been lingering concerns, however, that Ankara would continue to focus its military forces on Kurdish rebels, and not spend enough effort battling IS.

Two days ago, an IS propaganda video emerged, calling on Turks to rise up against Turkish President Recep Tayyip Erdogan, and pledged to conquer Istanbul.

The video comes at the height of a political crisis in Turkey, after Prime Minister Ahmet Davutoglu failed to form a coalition government following elections in June. A snap poll now looks likely in November.

mm/lw (AFP, AP, dpa)