Ruthless jihadists spearheading a Sunni militant offensive in Iraq have declared an "Islamic caliphate" and ordered Muslims worldwide to pledge allegiance to their chief, in a move the US dismissed as meaningless.

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 jihadist-led fighters in a swift advance that left nearly 2,000 people dead this month, 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 jihadist 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.

- 'Sense of ambition' -

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 jihadist.

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.

'No meaning'

Washington, however, said the caliphate declaration had "no meaning", with State Department spokeswoman Jen Psaki telling reporters that it just "exposed the true nature of this organisation and its desire to control people by fear."

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.

They have nevertheless suffered heavy casualties, with 380 soldiers among the nearly 2,000 people who died this month, the highest death toll since May 2007, according to figures released by Iraqi ministries.

In May 2007, Iraq was in the throes of a brutal sectarian war between the Shiite majority and the Sunni Arab minority that claimed tens of thousands of lives.

But Maliki's national reconciliation adviser, Amr Khuzaie, said the current crisis was even more dangerous.

"Now, the danger is definitely more... than 2006, 2007," he told AFP.

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.

Iraq: from jihadist onslaught to caliphate
Baghdad (AFP) June 30, 2014 - In just three weeks, jihadists have led a lighting advance by Sunni Arab militants through northern and north-central Iraq and declared an "Islamic caliphate" straddling Iraq and neighbouring Syria.

Their offensive has left more than 1,000 people dead, displaced hundreds of thousands and piled pressure on Iraq's Shiite Prime Minister Nuri al-Maliki.

The following is a timeline:

June 10

- Sunni Arab militants, led by jihadists of the Islamic State of Iraq and the Levant (ISIL), seize Iraq's second biggest city Mosul. Many security personnel simply abandon their weapons and uniforms and flee.

June 11

- The insurgents seize executed dictator Saddam Hussein's hometown Tikrit, capital of Salaheddin province.

- ISIL spokesman Abu Mohammed al-Adnani promises the militants will press their advance to Baghdad.

June 12

- Kurdish forces take over Kirkuk to protect the northern oil hub from the militants, who have taken over several mainly Sunni Arab towns in the west of the province.

June 13

- Top Shiite cleric Grand Ayatollah Ali al-Sistani urges Iraqis to take up arms against the militants advancing on the capital.

June 19

- The insurgents attempt to seize Iraq's biggest oil refinery outside the northern town of Baiji but, after 24 hours of heavy fighting, the army regains control.

- US President Barack Obama says Washington is prepared to send up to 300 military advisers to study how to train and equip Iraqi forces.

June 20

- Sistani says the jihadists must be expelled quickly from Iraq and that the next government must avoid "past mistakes", an apparent rebuke to Maliki, premier since 2006 and accused of sectarian policies towards the Sunni Arab minority.

June 23

- Insurgents overrun the strategic Shiite-majority northern town of Tal Afar and its airport.

June 27

- The US military starts flying armed drones over Baghdad.

June 28

- Thousands of Iraqi forces, backed by aircraft, launch an counter-offensive to retake Tikrit.

June 29

- ISIL declares an "Islamic caliphate" -- extending from Aleppo in northern Syria to Diyala province northeast of Baghdad. Renaming itself the Islamic State (IS), it declares its chief Abu Bakr al-Baghdadi "caliph" and "leader for Muslims everywhere".

June 30

- Iraqi troops press their fightback against Tikrit, but an officer acknowledges they are still only on the outskirts.