Leaders from the Islamic State [IS] and al-Qaida terror groups have agreed to stop fighting each other and work together to battle common enemies in northern Syria, according to a senior Syrian opposition official.

For over a year, IS and the al-Qaida-affiliated group Nusra Front have been battling each other to control the fight to overthrow Syrian President Bashar al-Assad, but leaders gathered at a farmhouse in northern Syria last week to put their differences aside.

If confirmed, the agreement between the extremist groups would be a blow to the US-led coalition conducting airstrikes against the terror groups in Iraq and Syria and arming more "moderate" rebels in the region.

According a Syrian opposition official speaking to the Associated Press, the meeting between the leaders occurred on 2 November in the town of Atareb, near Aleppo. They agreed to stop battling each other in order to tackle Kurds, western-backed Syrian rebels and the Assad regime.

The Associated Press also reported that members of the al-Qaida-linked Khorasan Group, rebel group Ahrar al-Sham and the IS-allied Jund al-Aqsa group were all present at the meeting.

Tom Joscelyn, an American terror analyst for the Long War Journal, told the Associated Press that information has been emerging "that would seem to fit in with that being what they were driving at. There has been a big push on the al-Qaida side to get this (alliance) through."

"If there is less blood being spilled against each other and they don't have to worry about that, that's going to make it easier for the jihadis to go after Assad or any Western-backed forces," he added.

IS had splintered from al-Qaida at the height of the Syrian civil war, disobeying orders from AQ leader Ayman al-Zawahiri not to operate independently from the Nusra Front.

Al-Qaida cut ties with the group earlier this year and released a statement to distance themselves from IS' brutality.

"We were not informed about [IS'] creation, nor counselled," al-Qaida said in a statement earlier this year. "Nor were we satisfied with it. Rather, we ordered it to stop. Isis is not a branch of AQ and we have no organisational relationship with it. Nor is al-Qaida responsible for its actions and behaviours."

However, last month, in a message entitled "A Statement on the Crusade Alliance" posted on Twitter, al-Qaida in the Arabian Peninsula implored mujahideen to stop infighting and come together to fight common enemies.

"We recommend that all the mujahideen forget their differences and stop fighting among themselves – and instead strive to push off this crusade that has been targeting everyone."

"This campaign is indeed a campaign against Islam, against the rule of God – and its intention is to keep the Muslims the way their enemies want them to be – with no command or anything of their own.