BEIRUT: About 70 US-trained rebels have returned to Syria after receiving training in Turkey, crossing the border in gun-mounted four-wheel drives, the United States (US) military's Central Command said Monday.

The development comes as US administration has been struggling to defend its military strategy in Syria, directed against the self-styled Islamic State (IS) militant group with an air campaign and programs to train, assist and equip local forces.

Last week, US Central Command spokesman Air Force Col. Patrick Ryder said that about 100 Syrian recruits were completing their US-led training programs and that most would soon be in Syria.

Central Command said in a news release that about 70 graduates of the Syria Train and Equip program had re-entered Syria with their weapons and equipment and were operating as New Syrian Forces alongside Syrian Kurds, Sunni Arab and other anti-IS forces.

The rebels' return was first reported by the Britain-based Syrian Observatory for Human Rights (SOHR).

In July, shortly after the first 54 US-trained fighters arrived, Syria's affiliate of Al Qaida known as the Nusra Front, or Jabhat Al-Nusra, attacked them, killing several and taking others hostage while many fighters fled.

Ryder said that those rebels largely disbanded — of the 54, one was killed; one is being held captive; nine are back in the fight; 11 are available but not in Syria; 14 returned to Syria but quit the US program and 18 are unaccounted for.

The head of the Observatory, Rami Abdurrahman, said the men returned on Friday night.

They had been scheduled to return earlier in the month and have been receiving training, by British, American and Turkish trainers, near the Turkish capital Ankara, he said.

He said the men returned in 12 gun-mounted four wheel drives, crossing at the border near the Turkish town of Kalis. He said they also each had a bag with personal weapons and life vests.

The new US trainees are likely meant to intensify military pressure on Raqqa, the self-declared capital of the so-called caliphate of the IS, which the IS established across much of Syria and Iraq.

The city is sometimes targeted by American-led coalition airstrikes, and occasionally also the Syrian government forces.

In addition to changing the role of the US-trained rebels, the Pentagon would scale back their numbers from the original target of 5,400 per year to a much smaller total, perhaps 500, the US officials have said.

The overhaul comes at a time when Russia, a strong backer of Syria's President Bashar Assad, has increased its military buildup in the country as the embattled leader faces setbacks on the battlefield at the hands of IS militants and other Islamic fighters.

In its fifth year, the war has claimed a quarter of a million lives, left a million injured and half of the country's pre-war population on the move, displaced at home or refugees.

Meanwhile, violence in Syria's northern city of Aleppo claimed 27 lives on Monday.

A missile attack by government forces killed at least 18 people and wounded dozens in a rebel-held neighbourhood of the city, the Observatory said, describing a surface-to-surface missile hit a market in Aleppo's Shaar neighbourhood.

The Local Coordination Committees said the attack targeted a street with clothing stores. It said 20 died and more than 40 were wounded.

It is not uncommon for activists to give different casualty figures in the immediate aftermaths of attacks.

The government later said a rebel shelling on the government-controlled Al-Midan neighbourhood killed 9 civilians, and wounded dozens.

Aleppo, once the commercial centre of Syria, has been divided since 2012, with government forces controlling much of western Aleppo and rebel groups in control of the east.

Shelling and air raids on the city have left hundreds of people dead over the past weeks.

Also Monday, Syrian State News Agency (SANA) and the Observatory said two car bombs went off in the northern Syrian town of Ras al-Ayn.

SANA said the two suicide bombings killed four people. The Observatory said the four killed included a member of the local Kurdish force.