Saying "there are no innocents in the heart of the lands of the Crusaders," an Islamic State (ISIS) group spokesman has urged sympathisers to launch attacks on civilians in Europe and the U.S. during the Muslim holy month of Ramadan, which is expected to start early next month.

Abu Mohammed al-Adnani encouraged lone wolf attacks in an audio message released late Saturday by the ISIS media arm al-Furqan, which was picked up in Cairo, Egypt, by the Associated Press.

Al-Adnani said ISIS sympathisers who are unable to travel to the group's self-declared caliphate in Syria and Iraq should launched their own attacks in the land where they reside "to win the great award of martyrdom."

"The tiniest action you do in the heart of their land is dearer to us than the biggest action by us ... there are no innocents in the heart of the lands of the Crusaders," he said.

Al-Adnani also responded to the statement made last week by Brett McGurk, the U.S. presidential envoy to the 66-country anti-ISIS coalition, who said that "this perverse caliphate is shrinking."

The ISIS spokesman said even if the jihadist group was pushed out of its strongholds in Raqqa and Mosul this would not count as defeat because "defeat is the loss of will and the desire to fight."

Last Saturday, Gen. Joseph Votel, the new commander of U.S. forces in the Middle East, was reported to have made a secret trip to Syria as part of the coalition's effort to develop credible Arab fighters to retake Raqqa.

Mosques in Raqqa broadcast an announcement that civilians would be allowed to leave the city, after planes thought to belong to the coalition dropped flyers instructing residents to leave ahead of an offensive.

41 dead in 2 ISIS bomb attacks in Yemen

Following the ISIS warning of lone wolf attacks in Western countries during the coming Ramadan, the jihadist group claimed responsibility for twin bombings against the Yemeni army which killed 41 people in second city Aden on Monday, the Telegraph reports.

In a statement posted online, the ISIS said one of its militants detonated an explosives belt among "apostate soldiers" at an army recruitment centre, followed by a bomb that exploded at the gate of the nearby Badr army base in Aden's Khormaksar district.

Aden is the headquarters of the government of Yemeni President Abedrabbo Mansour Hadi who is battling Shiite rebels who control the capital. It has been under attack from ISIS and Al-Qaeda forces for months now.

Iraq launches operation to recapture Falluja

Meanwhile, Iraqi Prime Minister Haider al-Abadi announced on Monday the start of an operation to recapture the city of Falluja from ISIS, CNN reports.

Abadi said Iraqi security forces would provide safe passage for civilians to leave the city during the operation.

Hours earlier, Iraq's military urged Falluja residents to flee their homes ahead of a major operation to recapture the city from the jihadist group.