The incident has raised security fears at Istanbul's Ataturk airport as no security camera footage was available to find those responsible for the mysterious blessings, airport news website Airporthaber reported.

The discovery of Arabic writing on a jet engine at Europe's fifth-busiest airport on Sunday led to fears among staff who, unable to read it, feared it might be linked to Islamic State militants, Airporthaber said.

Similar inscriptions were then found on the engines of three more aircraft, all of which had arrived from different destinations.

Initial fears that the writings were Islamist propaganda were unfounded, however. A spokesman for the company told Reuters on Wednesday that they were in fact an Arabic prayer.

"An investigation has been launched into this issue. We don't think there is a link with a terror organization, nor was there a threat to our flights," the spokesman added.

The incident comes as Islamic State fighters besiege the Syrian town of Kobani just meters from Turkey's southeastern frontier.

Foreign diplomats estimate that hundreds of so-called "foreign fighters" have traveled via Istanbul to reach the battlefields of Syria and Iraq, and say NATO member Turkey could be vulnerable to Islamist plots targeting western tourists.

Fear of retaliation from Islamic State has played a role in Turkey's reluctance thus far to take a frontline role in the U.S.-led coalition to tackle the radical group, which has captured vast swathes of territory in the Middle East, sending shockwaves through the region.