Notorious al-Qaida splinter group Islamic State in Iraq and Syria, or ISIS, has declared a "caliphate," a state ruled by strict Islamic law.

The announcement was made in a series of video statements and tweets over the weekend. ISIS also stated that it has renamed itself "Islamic State" and called on Muslims around the world to pledge allegiance to their organization.

"Accordingly, the 'Iraq and Sham' (Levant) in the name of the Islamic State is henceforth removed from all official deliberations and communications, and the official name is the Islamic State from the date of this declaration," group spokesman Abu Muhammad al-Adnani said in the statement, according to Reuters.

The group proclaimed leader Abu Bakr al-Baghdadi as Caliph, or head of state, the statement said.

"He is the imam and khalifah (Caliph) for the Muslims everywhere," the statement said.

The Sunni Muslim militant group seeks to recreate a medieval-style caliphate erasing borders from the Mediterranean to the Gulf. It deems Shiite Muslims to be heretics deserving death.

"It is incumbent upon all Muslims to pledge allegiance to (him) and support him. ... The legality of all emirates, groups, states and organizations becomes null by the expansion of the khalifah's authority and arrival of its troops to their areas," the statement said.

The move directly challenges al-Qaida central leadership, which has already disowned it, and conservative Gulf Arab rulers.

One of the group's videos features a fighter with an AK-47 who explains the new caliphate mostly in English.

"This is not the first border we will break; we will break other borders," an ISIS jihadist warned, The Guardian reported.

Later, the fighter vows that jihadists will free Palestine.

"We are not here to replace an Arab cahoot with a western cahoot. Rather, our jihad is more lofty ... . We are fighting to make the word of Allah the highest," the spokesman said. "There is no army in the world that can withstand the soldiers of Islam."

Abu Mohammed al-Adnani defined the Islamic state's territory as stretching from northern Syria to the Iraqi province of Diyala, northeast of Baghdad.