Initial indications suggest ISIS was responsible for twin bombings in the Turkish capital Ankara and investigations are focusing on the radical Islamist group, two senior Turkish security sources told Reuters on Sunday.

One of the sources said Saturday's bombings, which a pro-Kurdish opposition party said had killed 128 people, bore striking similarities to a suicide bombing in July in the town of Suruc near the Syrian border, also blamed on ISIS.

"This attack was in the style of Suruc and all the signs are that it was a copy of that attack ... the signs point to ISIS," the source said on condition of anonymity.

"All signs indicate that the attack may have been carried out by ISIS. We are completely focused on ISIS," the second source told Reuters.