LONDON — More than 100 Islamic prayer-leaders from various denominations of Sunni and Shiite Muslims have signed a letter calling on British Muslims not to travel to Iraq or Syria to fight.

"We urge the British Muslim communities to continue the generous and tireless efforts to support all of those affected by the crisis in Syria and unfolding events in Iraq, but to do so from the UK in a safe and responsible way," the open letter, released on Friday, says.

An estimated 500 British Muslims are believed to have taken up arms in Syria.

Security services say the influx of Western Muslims to Syria creates a threat to the West, when radicalized fighters return home from fighting in the region.

Britain has stepped up security at airports after U.S. officials said they were concerned that al-Qaida operatives in Syria and Yemen were developing bombs that could be smuggled onto planes.

Western officials are worried that the recent battlefield successes of the Islamic State in Iraq and the Levant (ISIL), an al-Qaida splinter group, have drawn a growing number of militants from America and Europe to the jihadist cause and they would have easy access to flights headed for U.S. cities.

"As we near the end of the first week of Ramadan our message is simple, we have come together to urge British Muslim communities not to fall prey to any form of sectarian divisions or social discord," said Qari Mohammed Asim, an Islamic prayer leader in the northern English city of Leeds.