U.S. President Barack Obama and Turkish counterpart Recep Tayyip Erdogan on Thursday vowed to step up the fight against the Islamic State group and strengthen Syria's moderate opposition.

Speaking by telephone in the wake of Saturday's bomb attack in Ankara that killed 99 people, and weeks before Obama travels to Turkey for a G20 summit, they discussed sometimes difficult cooperation on countering Islamic State in Syria.

Both countries have vowed to fight the extremists, but Turkish air strikes have largely focused on targeting Kurdish militia group the PKK.

Obama and Erdogan stressed the "urgent need for an end to PKK attacks in Turkey," the White House said, while Obama "offered condolences as well as US solidarity in the face of the security threats Turkey faces."

"The two leaders reaffirmed their shared goal of stepping up military pressure on ISIL (Islamic State) and strengthening moderate opposition elements in Syria to create conditions for a negotiated solution to the conflict, including a political transition."

Those efforts in Syria have been complicated by Russia and Iran's support for President Bashar al-Assad and Moscow's aerial bombardments of his foes, many of whom are not linked to Islamic State.