The Turkish military killed two suspected Islamic State members trying to enter Turkey illegally at the weekend, security sources said on Thursday, including a man thought to be plotting a suicide bomb attack in Turkey.

On Tuesday, three suicide bombers opened fire and then blew themselves up in Istanbul's main airport, killing 42 people in the deadliest of a series of suicide attacks in Turkey this year.

The security sources said one of the suspected militants killed at the weekend, a Syrian national, had flown from Damascus to the Kurdish-controlled border city of Qamishli on June 21. They published a photo of the used boarding pass.

The militant was thought to be planning a suicide attack in the Turkish capital or the southern province of Adana, home to Incirlik, a major base used by U.S. and Turkish forces through which some coalition air strikes against Islamic State are carried out.