* Al Qaeda-linked group loses base in Aleppo -monitoring group
* Heavy rebel infighting has raged since last week
* ISIL spokesman says fellow Islamists dragged into conspiracy (Updates death toll from rebel infighting)
By Erika Solomon
BEIRUT, Jan 8 (Reuters) - An al Qaeda affiliate based in Iraq and Syria has vowed to crush opposition groups it has been confronting in the worst outbreak of infighting among rebels since the start of the uprising against President Bashar al-Assad.
Heavy fighting between rebels in Syria has killed hundreds of people over the last five days and rattled the al Qaeda-linked faction, the Islamic State of Iraq and the Levant (ISIL).
The group lost its main base in the northern city of Aleppo to rival rebels on Wednesday, a monitoring group said.
The statement from the ISIL spokesman was issued late on Tuesday, hours after the head of the al Qaeda branch in Syria, the Nusra Front, called for a truce to halt the fighting.
The clashes have pitted many rebel groups in Syria, including Islamist forces, against ISIL and have been the bloodiest rebel-on-rebel violence since the civil war in Syria began in the first half of 2011.
"Crush them completely and kill the conspiracy in its cradle," said ISIL's spokesman, known as Abu Mohammed al-Adnani.
While both the Nusra Front and ISIL have roots in the global al Qaeda network and welcome foreign militants, the Nusra Front has focused its goals on toppling Assad instead of creating an Islamic state, which has been the main aim of ISIL.
Nusra forces have also cooperated more with other rebel groups and largely avoided the power struggles that ISIL has faced since wresting control of many opposition-held areas from other groups.
ISIL is the restructured al Qaeda branch of Iraq, but its spread into Syria is opposed by al Qaeda's central leadership, which has recognised the Nusra Front instead.
Adnani said fellow Islamists from the Sunni Muslim sect had been dragged into a conspiracy against it.
"Those who are from battalions raising the flags of Islam, who tricked you? Who implicated you and made you sign to fight against the mujahideen (holy warriors)?"
"PLUCK THE HEADS"
In Adnani's audio message, he also told ISIL fighters to "pluck the heads" of any leaders of the Syrian National Coalition - the internationally recognised opposition - or rebel groups tied to them.
"Kill them wherever you find them and without dignity," he said. "They launched this war against us and started it ... Therefore anyone who is a member of this entity is a legitimate target for us wherever he may be, unless he declares his innocence from this sect in public and renounces his fight against the mujahideen."
It was impossible to verify the authenticity of the statement, but it was widely cited on Islamist social networks on the Internet.
Rebel groups last week launched what appeared to be a series of coordinated strikes against ISIL in northern and eastern Syria after months of increasing tensions with the group, which has alienated many Syrians in rebel-held regions. The fighting was endorsed by the opposition's mainstream National Coalition, which is backed by Western and Gulf states.
Islamist rebels on Wednesday took control of ISIL's main base in Aleppo, a northern city that was once Syria's largest and is now divided between government and rebel forces, the Britain-based Syrian Observatory for Human Rights said.
Some 385 people have been killed in the rebel infighting since Friday morning in Aleppo, Raqqa, Idlib and Hama provinces, according to the Observatory.
Fifty-six of those killed were civilians - nine executed by ISIL and the others caught in crossfire - while 131 were ISIL fighters and 198 were from rival factions, it added.
The ISIL base was located in a children's hospital, the opposition monitoring group said, adding it was unclear what happened to the hundreds of fighters based there but dozens of prisoners were reported to have been freed. (Reporting by Erika Solomon; Additional reporting by Alexander Dziadosz; Editing by Will Waterman and Giles Elgood)