KUALA LUMPUR - Three Malaysian women have allegedly travelled to the Middle East as "comfort women" for the Islamic State of Iraq and Syria (ISIS) fighters, according to senior intelligence officials.

Jihad Al-Nikah, or sexual jihad, refers to women joining the jihad by offering sex to fighters to boost their morale.

"These women are believed to have offered themselves in sexual comfort roles to ISIS fighters who are attempting to establish Islamic rule in the Middle East.

"This concept may seem controversial but it has arisen as certain Muslim women here are showing sympathy for the ISIS struggle," an intelligence official, who spoke on condition of anonymity, told The Malaysian Insider news site.

One of the women, who is in her 30s, had travelled to Turkey in December last year to meet up with middlemen who helped her to complete her journey to Syria via land routes, said the official.

Another Malaysian woman in her 40s is believed to have linked up with ISIS forces sometime in April this year, the official added.

Besides the three Malaysians, Sunni Muslim women from Australia and the United Kingdom have also joined up with ISIS, the Insider reported.

Over 600 British Muslims are believed to be fighting for ISIS. The number also includes "British Muslim women who are not fighting on the front line, but are still involved by performing sexual jihad", the intelligence official said.

The official added that "there may be up to 50 Malaysians in the Middle East".

So far, three Malaysians have reportedly died in the Iraq-Syria struggle. Among those dead are Malaysia's first suicide bomber, 26-year-old factory worker Ahmad Tarmimi, and 52-year-old jihadist Mat Soh from Selangor.

ISIS, which overran swathes of northern Iraq and eastern Syria in June and declared a caliphate, has become a top national security preoccupation of the Obama administration after the beheading of US journalist James Foley.

On Wednesday, Malaysia Prime Minister Najib Razak denounced ISIS's mass executions and killing of civilians, calling the extremists' actions a "counter to our faith".

“When states fail, or are broken by war, extremists see opportunity. They prey on the differences between people, using faith as a cover for atrocities. But they do not speak for us.

“The actions of the Islamic State militants in Syria and Iraq are counter to our faith, our culture, and our common humanity. They are against the teachings of the Prophet, a man of peace and moderation; and against Islamic law, which prizes the protection of life above all," he said in a statement.