BEIRUT - Syrian rebels, including the main Islamist factions, said Monday the creation of a caliphate by the Islamic State (IS) was "null and void".

"We see that the announcement by the rejectionists of a caliphate is null and void, legally and logically," the groups said in a statement, using a pejorative term to refer to the extremist Islamic State.

Among the signatories were the Islamic Front, Syria's biggest rebel coalition, and Majlis Shura Mujahideen al-Sharqiya, an alliance in the eastern province of Deir Ezzor near Iraq, that includes the Al-Qaeda affiliate Al-Nusra Front.

The statement, signed by the religious bodies of each rebel group, went on to say that the Islamic State's announcement "changes nothing in terms of how we perceive them, or how we will deal with them".

The statement also "warns Muslims and all jihadist factions from putting their capabilities" at the service of the IS.

The signatories of the statement have all been at war against IS -- previously known as the Islamic State of Iraq and the Levant -- since January.

The rebels said the announcement of a "caliphate" was divisive and a bid to "abort the blessed revolutions in Syria and Iraq".

In Iraq, where IS was originally rooted, the jihadist group has in recent weeks spearheaded a lightning offensive, seizing large swathes of territory.

In 2013, Syria's rebels initially welcomed IS among their ranks in their bid to oust President Bashar al-Assad, but they turned against the jihadist group because of its systematic abuses and its quest for hegemony.

Their statement comes a day after the ruthless Islamic State declared a "caliphate" straddling Iraq and Syria, declaring its chief Abu Bakr al-Baghdadi "the caliph" and "leader for Muslims everywhere".

Syria's war began as a peaceful movement demanding Assad's ouster but morphed into a war after the regime unleashed a brutal crackdown on dissent.

Many months into the violence, the war began to attract jihadists.

IS has kidnapped thousands of Syrians, including rebels and opposition activists, and carried out daily summary, public executions in areas under its control.

It controls Raqa province in northern Syria, large areas of oil-rich Deir Ezzor on the Iraq border, and parts of Aleppo in the north.

Its Iraq offensive has boosted its confidence, partly because it has captured large amounts of heavy weapons, including US-made armoured vehicles, from fleeing Iraqi troops.

Activists in Raqa on Monday reported a major military parade by IS troops in the city, during which they showed off heavy weapons -- including missiles -- captured in Iraq and then transported into Syria.