A Sunni armed group which controls large areas of Iraq and Syria has announced the establishment of a "caliphate" straddling the two countries, urging other groups to pledge allegiance.

In an audio recording released on Sunday, the group formerly known as the Islamic State of Iraq and the Levant declared its chief, Abu Bakr al-Baghdadi, "the caliph" and "leader for Muslims everywhere".

"The legality of all emirates, groups, states and organisations becomes null by the expansion of the caliph's authority and the arrival of its troops to their areas," said the group's spokesman Abu Mohamed al-Adnani. "Listen to your caliph and obey him. Support your state, which grows every day.''

The group announced that it was now called the "Islamic State". According to the statement, the new caliphate stretches from Iraq's Diyala province to Syria's Aleppo.

"The Shura [council] of the Islamic State met and discussed this issue [of the caliphate] ... the Islamic State decided to establish an Islamic caliphate and to designate a caliph for the state of the Muslims, said Adani.

"The words 'Iraq' and 'the Levant' have been removed from the name of the Islamic State in official papers and documents."

An official document was also released, in English and several other languages.

It describes Baghdadi as "the sheikh, the fighter, the scholar who practices what he preaches, the worshipper, the leader, the warrior, the reviver, descendant from the family of the Prophet, the slave of God".

It urges Muslims to "gather around your caliph, so that you may return as you once were for ages, kings of the earth and knights of war."

Al Jazeera's Imran Khan , reporting from Baghdad, said that a caliphate was effectively an Islamic republic led by one leader, regardless of national boundaries.

He added however that Sunni groups which have fought with the Islamic State group in Iraq have yet to pledge allegiance.

The announcement might cause problems with other Sunni fighters in Iraq, who are fighting the central government of Nouri al-Maliki and not for the caliphate, our correspondent said.

The group's announcement came as Iraq's army pressed its offensive to recapture the northern city of Tikrit.

Troops backed by helicopter gunships began an assault on the city on Saturday, while tanks and helicopters were sent to areas around Tikrit university on Sunday, security sources said.