Obama, Erdogan vow to intensify fight against IS

US President Barack Obama and Turkish counterpart Recep Tayyip Erdogan vowed to step up the fight against the Islamic State (IS) 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 the often difficult cooperation on countering IS in Syria.

Both countries have vowed to fight IS and other such groups, but Turkish airstrikes have largely focused on targeting the Kurdish militia group, 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 IS.

Turkish President Recep Tayyip Erdogan on Tuesday ordered a special state probe into possible shortcomings over the recent Ankara bombings, saying intelligence indicated the attack had its origins in Syria.

The government has said IS is the prime suspect behind Turkey's deadliest attack in modern history, which saw two suicide bombers blow themselves up in crowd of leftist and pro-Kurdish activists at a peace rally.

There appear to have been two suicide bombers, both male, working some distance apart and timing their detonations so that there was a stampede towards the second bomber to maximise the number of deaths. They also concentrated on those sections of the crowd where supporters of the left-of-centre, Kurdish-rooted People's Democratic Party (HDP) were most numerous. Like the Suruc massacre which killed 34 people on 20 July in southeastern Turkey, the attack was focused on Kurdish groups and their left-wing Turkish allies.

The deaths of more than 100 peace demonstrators has convulsed Turkey like no other event in its recent history, bringing demonstrators out onto the streets not only in Ankara and Istanbul but in other towns across the country.