Hundreds of fighters under the command of the opposition Free Syrian Army (FSA) have reportedly switched allegiance to al-Qaeda-aligned groups, in a move described as a huge blow to moderate rebel forces.

Activists and military sources have told Al Jazeera that the 11th Division - one of the biggest FSA brigades - has switched allegiance to the al-Nusra Front in Raqqah province, a border province with Turkey.

A video was uploaded to YouTube on Thursday purporting to show members of the 11th Division parading through Raqqah with Nusra fighters.

In the video clip, a voice can be heard saying in Arabic, "Raqqah ... September 19, 2013 ... The convoy of Nusra ... God is great ... Nusra in Raqqah province."

The switch, if confirmed, tightens Nusra's control of Raqqah just days after the Islamic State of Iraq and the Levant (ISIS) attacked members of the Free Syrian Army in Azaz, on the border with Turkey.

The Reuters news agency, citing sources inside Syria, also reported that entire units of the FSA had joined Nusra and the ISIS in recent days.

The Raqqah Revolutionaries - which is part of the 11th Division - has about 750 fighters in total, according to a source close to al-Qaeda linked forces.

Abdulhamid Zakarya, military spokesman of Chiefs of Staff of the FSA, denied that Division 11 had joined Nusra. However, he said it had signed an agreement to collaborate in military operations.

In a separate statement, the FSA also condemned the ISIS for its actions in Azaz, saying it was going against the principles of the Syrian revolution.

“ISIS no longer fights the Assad regime. Rather, it is strengthening its positions in liberated areas at the expense of the safety of civilians. ISIS is inflicting on the people the same suppression of the Baath party and the Assad regime.”

Anita McNaught, Al Jazeera's correspondent in Antakya in neighbouring Turkey, said that if proved true, the switches of allegiance would be a serious blow to the FSA's strength, and could have significant implications outside of Syria.

The US State Department designated Al Nusrah Front a terrorist organisation on 11 December 2012. There are financial sanctions in place.

"This means that the FSA has suddenly lost serious amounts of loyal fighters ... it's basically being swallowed up by Nusra," she said, adding that it would be very difficult for the West to support a rebel army dominated and commanded by al-Qaeda linked groups.