The men entered northern Syria's Aleppo province between Friday night and Saturday morning, said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.

He added that the rebels had crossed into Syria with 12 vehicles equipped with machine guns and had been given air cover from the US-led coalition, which is carrying out air raids on "Islamic State" (IS) targets.

The fighters received combat training from US and coalition forces in neighboring Turkey before being deployed to the war-torn country. Their entry point is a rebel supply route, which has regularly been targeted by the militant Islamist group.

Abdel Rahman said the fighters have been deployed to support two US-backed units.

The rebels are part of a much-criticized program to train up to 5,400 Syrian rebels a year over the next three years, as part of Washington's strategy to have local partners help combat IS militants.

But the first batch of less than 60 fighters came under immediate attack from al Qaeda's Nusra Front after arriving in Syria. Most were either captured, killed or fled.

Training scrutinized

A top US general told Congress on Wednesday that only four or five rebels trained under the program were still fighting in Syria.

The revelation led to deep criticism of defense officials over America's overall strategy against IS. Despite dozens of daily air strikes, the jihadist group still controls large parts of Iraq and Syria and has affiliates in other fragile Middle Eastern countries. One US politician labeled the program a "joke."

US officials said they were planning a major overhaul of the deployment strategy. Instead of preparing rebels for frontline combat, they plan to embed them with established Kurdish and Arab forces fighting in Syria.

The $500 million (442 million euro) program has so far trained less than 150 people, after many would-be recruits failed the screening process.

A similar strategy in Iraq has recruited 13,000 soldiers and more than 3,000 others are currently in training.

The Pentagon has acknowledged that the fight against IS is likely to take several years.

mm/rc (AFP, AP, Reuters)