Fighters from several Syrian rebel groups have seized the headquarters of the Islamic State of Iraq and the Levant in Aleppo, the Syrian Observatory for Human Rights says.

The UK-based watchdog said on Wednesday that ISIL had been using a disused hospital in the Qadi Askar area of the city, although it was unclear what had happened to the fighters who had been operating from there.

The development marks a setback for ISIL, which late on Tuesday had urged people to target those who opposed them.

In an audio clip, ISIL spokesman Abu Mohamed al Adnani al Sham said: "The Syrian National Coalition and the Free Syria Army military council declared war on ISIL.

"All their members are legitimate targets. We have placed rewards on their heads. Kill them wherever they are found.

"Know that we have armies in Iraq and Syria ... we warn you. We will crush them and kill the conspiracy in its

cradle.''

Wednesday's takeover is a victory for rebels battling al-Qaeda allies in Syria, but the movement to remove President Bashar al-Assad from power remains in disarray, with groups fighting each other for control of the north.

Al Jazeera's Zeina Khodr, reporting from Beirut in neighbouring Lebanon, said: "Clashes in the rebel-held north weakens the opposition in its fight against the regime, but in the long term the opposition may benefit from this battle.

"Al-Qaeda's presence in Syria has been one of the reasons for the international community's reluctance to militarily support the rebels and the regime has been able to portray the fight as one against terrorism.

"The opposition has denied that it turned its guns against al-Qaeda to appease the West. It says it did so because the people of Syria could no longer tolerate the abuses by the foreign fighters."

At least 300 people have been killed in the infighting in Syria during the past five days, according to the Syrian Observatory for Human Rights.