American warplanes carried out airstrikes to aid US military-trained Syrian rebels who recently came under attack from fighters belonging to an al-Qaeda-linked terror group.

The rebels came under heavy fire in northern Syria and asked the US military to intervene—the first time they called in US airstrikes, NBC News wrote.

According to US military officials, the Free Syrian forces were stormed by around 50 al Nusra fighters who are affiliated with the al-Qaeda.

The siege forced the Syrian moderates to make a desperate plea for help to the US military.

American warplanes immediately responded and carried out airstrikes to repel the enemy forces.

US officials declined to say how many the US-trained rebels were or where the attack was held.

Under a loose alliance with the moderate Syrian rebels, the US has agreed to provide combat support to aid them.

US military officials consider what they call the "train and equip" mission in Syria as crucial to the campaign to weed out the Islamic State militants in the area. However, the officials said they are short of the 5,400 fighters they hoped to train by yearend.

The US has only trained 54 Syrian rebels so far, arming them with only small weapons, officials said.

Another "couple hundred," however, are waiting to be trained by the US military, they added.

US defence and military officials also rejected reports that al Nusra has captured or killed US-trained Syrian rebels in the attack on Thursday in northern Syria.

Officials did confirm, however, that the leader of the 30th Division, Nadim Hassan, and an unknown number of Syrian rebels were captured by al Nusra fighters on Thursday.

The 30th Division is one of the more moderate Syrian rebel groups. The US has already considered training some of their forces but has not yet done so.

Al Nusra, in a statement, said they caught a number of US-trained fighters and warned others against joining what they described as "the American project," the BBC reported.

The 30th Division, meanwhile, said in a separate statement that five of their fighters were killed in the clashes.