Fears Australian jihadists slipped out of the country

There are fears that Australian jihadists have used the recent pilgrimage to Mecca as a cover to slip out of the country and fight for terrorist groups overseas.

About 5000 Muslims are believed to have left Australia last month for the annual Haji pilgrimage, which took place in early October.

Security agencies will launch an investigation next month to identify any Islamic extremists who may have left the country to fight for Islamic State and other terror groups in Syria, News Corp Australia reports.

Authorities won't know for several weeks whether Muslims who left for Mecca are taking holidays or are involved in something more sinister, a senior security source told News Corp.

Security agencies are reportedly set to investigate each Australian who departed the country with the intention of attending the Haji pilgrimage and who has not yet returned.

About 60 Australians are fighting with Islamic State in Syria and Iraq, with at least 100 others offering support and 70 who have had their passports cancelled on security grounds.