The group, which calls itself the Islamic State of Iraq and Syria, has lost ground in recent fighting with other rebel forces opposing the regime of Syrian President Bashar Assad, the New York Times reported.
Another al-Qaida-linked group that has been fighting the Islamic State, the Nusra Front, unveiled a plan Tuesday to end the bitter conflicts between rebel groups.
In a recording posted online, Abu Mohammed al-Jolani, the head of the Nusra Front, said the "incorrect policies" of Islamic State caused the infighting. He called for a cease-fire and creation of an Islamic court to settle differences.
The authenticity of the statement was not immediately confirmed.
More than 270 people have died in four days of fighting as of Monday, said the London-based Syrian Observatory for Human Rights, which reported the seizure of the Islamic State headquarters. The opposition group, which has contacts in Syria, said the dead included 99 Islamic State fighters, 129 rebels and 46 civilians.