The Islamic State of Iraq and Syria (ISIS) today declared the restoration of an Islamic caliphate in the areas of Iraq and Syria it occupies and called on al Qaeda and other Sunni militant forces in the area to join them in acknowledging “a new era of international jihad."

ISIS's chief spokesman Abu Mohammed al-Adnani announced the militant group would now be called simply "The Islamic State" and that the group's leader, Abu Bakr al-Baghdadi, is to be instated as the state's new Caliph.

The announcement comes as US and Iraqi military forces battle to reclaim the strategic city of Tikrit from ISIS and other Sunni insurgents who overran the urban center earlier this month as part of an advancing offensive across parts of northern and western Iraq.

Backed by helicopters and tanks, Iraqi troops entered Tikrit on Saturday, but were reportedly pushed back by rebels forces.

ISIS, which initially sought to control vast areas of Syria and topple Syrian President Bashar Assad, has long since moved to establish a strict Shariah law-governed caliphate over the Middle East and oust Iraqi Prime Minister Nouri al-Maliki's Shiite-led government.

As Sunni insurgents continue to circle towards Baghdad and seize parts of the country, ethnic Kurdish militia have fought to keep ISIS at bay in parts of Kurdistan that lie in Iraq's north and called for al-Maliki to step down.

Today, Israel's leader backed Kurdish aspirations to establish independence, saying the escalating Iraqi crisis provided an opportunity for "enhanced regional cooperation."

Kurds "are a nation of fighters and have proved political commitment and are worthy of independence," Prime Minister Benjamin Netanyahu said in a policy speech.

His comments are at odds with the wider international community, including the US, which believes in maintaining a unified Iraq.