Some rebel groups and Islamic scholars in Syria have rejected the announcement of a so-called Islamic caliphate by an increasingly powerful rival.

Nine groups, including fighters and religious scholars, have dismissed the statement by Islamic State, an al-Qaeda offshoot which declared its leader ruler of the Muslim world earlier this week after military gains in neighbouring Iraq.

The group, formerly called the Islamic State in Iraq and the Levant (ISIL), has seized parts of eastern Syria and western Iraq but is still battling rival armed groups in Syria.

"The terms of the caliphate have not been realised at present, especially in terms of state organisations," the statement by the nine groups said, calling on Muslims to avoid siding with the Islamic State.

They also said that the announcement would be used as a pretext by foreign powers who want to tilt the balance against rebels seeking to topple Syrian President Bashar al-Assad and would improve his image in the West as a legitimate leader.

The June 30 statement, which Syrian activists said appeared genuine, was signed by groups including Islamic Front, a coalition which includes Saudi-backed combatants, and by the top body of Syrian Islamic scholars in exile.

Ongoing infighting

Al-Qaeda's official Syrian branch, the Nusra Front, one of Islamic State's main rivals in the country, has yet to comment.

Infighting between anti-Assad armed groups has killed about 7,000 people so far this year according to the Syrian Observatory for Human Rights, a monitoring group.

Islamic State, which derives much of its strength from foreign fighters, has seized large areas of Iraqi territory including the city of Mosul and has threatened to advance towards the capital, Baghdad.

In Syria, it is fighting with rivals in the province of Deir al-Zor, bordering Iraq. The Observatory said on Tuesday that Islamic State was in control of most of Albu Kamal, a border town where rivals have fought back in recent days.

Islamic State is an al-Qaeda splinter group that has roots in Iraq and expanded into Syria shortly after the start of the uprising against Assad three years ago.

It controls large parts of Syria's eastern oil-producing Euphrates River region, and its gains in Iraq since June 10 mean it has command of a cross-border expanse of territory.

By claiming leadership across the Muslim world - in a direct challenge to al-Qaeda's leader Ayman al-Zawahri - Islamic State could deepen divisions and rivalry among al-Qaeda offshoots.

"This announcement will only deepen the conflict with the central al-Qaeda organisation over the legitimacy of who represents it now and as a result who represents true Islam," said Jordan-based Hassan Abu Haniyah, an expert on the groups.