The Islamic State group has executed 100 of its own foreign fighters who tried to flee their headquarters in the Syrian city of Raqqa, the Financial Times reported on Saturday.

An activist opposed to both IS and the government of Syrian President Bashar al-Assad told the daily that he had "verified 100 executions" of foreign IS fighters trying to leave the group's de-facto capital.

IS fighters in Raqqa said the group has created a military police to clamp down on foreign fighters who do not report for duty. Dozens of homes have been raided and many fighters have been arrested, the FT reported.

Some fighters have become disillusioned with the realities of fighting in Syria, reports have said.

According to the British press in October, five Britons, three French, two Germans and two Belgians wanted to return home after complaining that they ended up fighting against other rebel groups rather than the Syrian army. They were being held prisoner by IS.

In total, between 30 and 50 Britons want to return but fear they face jail, according to researchers at the International Centre for the Study of Radicalisation at King's College London, which had been contacted by one of the fighters speaking on their behalf.

Reports emerged earlier this month of an Indian fighter who returned home to Mumbai after IS fighters made him clean toilets and do other menial jobs like fetching water.

Engineering student Areeb Majeed, 23, who travelled to Iraq with his three friends to join IS in May, was arrested and charged by India's National Investigation Agency with terror-related offences when he returned, according to the Press Trust of India news agency.

Since a US-led coalition began a campaign of air strikes against IS in August, the group has lost ground to local forces and seen the number of its fighters killed rise significantly.

In recent weeks, there have been a string of apparent setbacks for IS.

Iraqi Kurds claimed Thursday to have broken a siege on a mountain where Yazidi civilians and fighters have long been trapped.

The Kurdish advances came during a two-day blitz in the Sinjar region involving 8,000 Peshmerga fighters and some of the heaviest air strikes since a US-led coalition started an air campaign four months ago.

Meanwhile Thursday, the Pentagon said several IS leaders had been killed in US air strikes.

In 40 days across October and November, some 2,000 air raids killed more than 500 people, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group, which relies on a network of sources on the ground.