BAGHDAD - Ruthless militants spearheading an offensive in Iraq have declared an "Islamic caliphate" and ordered Muslims worldwide to pledge allegiance to their chief, in a spectacular bid to extend their authority.Iraqi forces meanwhile pressed a counter-offensive Monday against executed dictator Saddam Hussein's hometown of Tikrit, one of a string of towns and cities overrun by militant-led fighters in a swift advance that left more than 1,000 people dead, displaced hundreds of thousands and piled pressure on Prime Minister Nuri al-Maliki.Maliki's bid for a third term in office has been battered by the offensive and he is no longer seen as the clear frontrunner when the new parliament elected in April holds its opening session on Tuesday. A security source based near Tikrit said reinforcements had arrived with tanks and artillery on Monday. An army officer said troops controlled parts of the outskirts of the city, some 160 kilometres (100 miles) north of Baghdad, which the militants captured on June 11.The Islamic State of Iraq and the Levant renamed itself simply the Islamic State (IS) and declared its shadowy frontman the leader of the world's Muslims, in a clear challenge to Al-Qaeda for control of the global militant movement.IS announced on Sunday that it was establishing a "caliphate" - an Islamic form of government last seen under the Ottoman Empire - extending now from Aleppo in northern Syria to Diyala province in eastern Iraq, the regions where it has fought against the regimes in power.In an audio recording posted online, the group declared its chief Abu Bakr al-Baghdadi "the caliph" and "leader for Muslims everywhere". Henceforth, the group said, he is to be known as "caliph Ibrahim" - a reference to his real name.Although the move may not have immediate significant impact on the ground, it is an indicator of the group's confidence and marks a move against Al-Qaeda - from which it broke away, analysts say."I don't think this materially changes anything," said Shashank Joshi, research fellow at the Royal United Services Institute think tank in London. "What it really changes is the sense of their ambition. It's a potentially inspiring and invigorating movement for people worldwide."It will tempt many radicalised Muslims to join their cause."Baghdadi, thought to have been born in the Iraqi city of Samarra in 1971, is touted by the group as a battle-hardened tactician who fought American forces following the US-led invasion of 2003, and is now widely seen as rivalling Al-Qaeda chief Ayman al-Zawahiri as the world's most influential militant.His group has drawn thousands of foreign fighters, attracted by a combination of Baghdadi's own appeal, IS's efforts to establish what it believes is an ideal Islamic state, and the group's sophisticated propaganda apparatus, which publishes magazines and videos in English and a host of European languages.The group is known for its brutality, summarily executing its opponents and this week crucifying rival Islamist rebels in Syria.In Syria, IS fighters control large swathes of territory in Deir Ezzor near the Iraq border, Raqa in the north, as well as parts of neighbouring Aleppo province.In Iraq, it has spearheaded a lightning advance since June 9, capturing sizeable territories in the north and west, including the country's second city Mosul.Iraqi forces initially wilted in the face of the onslaught but have mounted an ambitious counter-offensive to take back Tikrit, a battle which could be crucial tactically and for the morale of the security forces.World leaders and leading clerics have pressed Iraqi leaders to unite and quickly form a government, but despite the urgency, politicians have warned that the process of choosing a new prime minister could take more than a month.Though Maliki emerged from the April 30 election in pole position to retain his post, with his bloc winning by far the most seats albeit not a majority, his opponents have stepped up their criticism of him in the wake of the offensive.