The US Department of Defence has started paying Syrian moderates a monthly stipend of $400 each as it employs their services to help fight the terror campaign being waged by the Islamic State, US military sources said.

The first group of about 90 Syrian fighters, who has started their training in May, will each be paid an allowance of $250 to $400 monthly.

"We can confirm that Syrian train and equip members will receive stipends while members of the program," said Navy Cmdr. Elissa Smith, a Pentagon spokesperson.

The stipend, which follows wages in pre-war Syria, will depend on the fighters' skills performance and leadership, according to Smith.

The Pentagon expects to have trained 3,000 fighters by the end of the year, said Smith, with the goal of having 5,400 able-bodied fighters by 12 months.

"For operational security, we will not announce when coalition-trained Syrian opposition forces will enter into Syria," she said.

Nearly 6,000 Syrians have signed up for the training, of whom 4,000 are waiting to be vetted. The recruitment process is still ongoing.

The training of the recruits, considered critical" by US Defence Secretary Ashton Carter, is expected to take several months.

The fielding of trained forces into Iraq to fight against the ISIS has been slow as the extremist group, which seeks to carve out its own caliphate in the region, has spread terror in northern Iraq and eastern Syria, taking major cities.

"Our train-and-equip mission in Syria has been challenging," Carter said. "But the requirement for a capable and motivated counter-ISIL ground force there also means we must persist in our efforts."

Trained Syrian fighters will be given small arms and taught to maneuver and communicate.

They will assume primary responsibility of protecting their communities against the ISIS' campaign. They are also foreseen to be ready to be deployed against the forces of President Bashar Assad. Syria's civil war has left 220,000 people dead and forced millions out of their homes.

"They're not fighting a large army," said David Phillips, director of the Peace-Building and Rights Program at Columbia University of the Syrian fighters. "Even small numbers can be effective on the battlefield. Nobody envisions this to be an easy or quick win. Developing a nucleus of capable fighters is the right way to start."

The Pentagon expected to put 24,000 new Iraqi security forces in the field by fall, but only 9,000 have so far enlisted for the training, according to Carter.

Iraqi forces have seized cities such as Tikrit from ISIS' hands, but they suffered their worst defeat when they were expelled without a fight from Ramadi in May.