ISTANBUL (Reuters) - 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.

The suspected militants were "neutralized" on June 25 after refusing the army's warnings at the Syrian-Turkish border while trying to cross, the sources said.

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.

(Reporting by Tulay Karadeniz; Writing by Humeyra Pamuk; Editing by Daren Butler and Janet Lawrence)