The United States has begun training moderate Syrian rebels as part of an effort to build a force capable of fighting against the Islamic State of Iraq and the Levant (ISIL) group.

Ash Carter, the US defence secretary, said on Thursday that the US military had begun training around 90 Syrian fighters in Jordan.

"Combat training has begun for a company-sized group from the new Syrian forces," Carter told reporters. "This programme is critical and a complex part of our counter-ISIL efforts."

The US plans to train and arm a force that is expected to eventually total more than 15,000 troops. So far, more than 3,750 Syrian fighters have volunteered for the training, and about 400 have completed the pre-screening.

The rebels, who come from several moderate groups in Syria, will get training on basic military equipment and skills, including firearms, communications and command and control abilities.

Part of the strategy, according to documents seen by the Reuters news agency, is to pressure President Bashar al-Assad by steadily increasing the opposition's prowess and territory under its control.

Proponents of the US military programme note Assad is already facing growing pressure after government forces endured a series of setbacks on the battlefield and ISIL fighters edge closer to Assad's stronghold in the coastal areas.

General Martin Dempsey, the top US military officer, said he believed Assad's "momentum has been slowed".

Some rebels sceptical

However, the programme faces deep scepticism, including from rebels fighting inside Syria. Some rebel leaders say the force risks creating divisions and cannot succeed without directly targeting Syrian government forces.

The US Congress has approved $500m to train about 5,000 Syrian fighters in a year's time.

About 1,000 US troops are expected to help with the training and 450 forces from the US-led coalition have already deployed for the effort, officials said.

Turkey has said it will start training rebels on its soil this weekend, with Saudi Arabia, Qatar and Turkey also expected to train the moderate fighters.

The US launched targeted air strikes against ISIL in Iraq last August, and in September expanded the campaign into Syria.

The group has declared a 'Caliphate' in territory across much of Iraq and Syria, and its rule has been marked by a brutal campaign of mass murders, beheadings, torture and slavery.