Dozens of fighters from the al-Qaida offshoot in Syria, the Jabhat al-Nusra front, have defected and joined Isis (Islamic State) in Syria in the past days, according to a monitoring group.

The London-based Syrian Observatory for Human Rights have reported that 40 militants from Syria's largest al-Qaida group moved to Al-Bab city, one of the bastions of the Islamic State northeast of Aleppo, and pledged allegiance to self-styled caliph Abu Bakr al-Baghdadi.

Isis reportedly offered an enslaved woman named Sabbieh to the leader of the 40 Nusra defectors as a reward for joining the Islamic State.

While no additional details were available on the desertion, the Observatory said the defected included an Emir who retired from fighting on the al-Nusra side.

Al-Nusra, which takes orders directly from al-Qaida's leader Ayman al-Zawahiri, has battled IS in Syria in the past year while becoming the strongest and best equipped among the rebel groups fighting against troops loyal to president Bashar al-Assad.

Since September, however, US-led air strikes on IS positions near the Syrian Kurdish town of Kobani, which has been besieged by the jihadists, led al-Nusra to move towards a new alliance with IS. The Islamist group called the attacks a "war on Islam" and at least 73 members defected to IS earlier in September.

A source said al-Nusra and IS leaders were holding war planning meetings.

In February, in a bid to reassert influence among rival Islamic groups in Syria, al-Qaida had severed ties with IS, who reportedly disobeyed orders from network leader al-Zawahiri not to operate independently from the official offshoot, the al-Nusra front.