Baghdad (AFP) - Prime Minister Nuri al-Maliki offered a general amnesty Wednesday in a rare conciliatory move to undercut support for militants whose offensive has overrun swathes of territory and threatens to tear Iraq apart.

The offer comes after a farcical opening to the new parliament, despite international leaders urging Iraq's fractious politicians to unite to help combat insurgents, as the military struggles to seize the initiative on the ground.

International leaders have warned Iraq's politicians there was no time to waste, while the head of a powerful jihadist group that led the militant advance urged skilled professionals to flock to help its newly proclaimed pan-Islamic state.

Maliki's surprise move, made in his weekly televised address, appeared to be a bid to split the broad alliance of jihadists, loyalists of executed dictator Saddam Hussein and anti-government tribes that has captured large chunks of five provinces, displacing hundreds of thousands of people.

"I announce the provision of amnesty for all tribes and all people who were involved in actions against the state" but who now "return to their senses", excluding those involved in killings, Maliki said.

It was not immediately clear how many people the amnesty could affect, but analysts have said some form of political reconciliation will be necessary to convince Sunni Arabs angry with the Shiite-led government to turn against their co-religionists and jihadists.

The vast majority of Iraq's Sunni Arab minority do not actively support the Islamic State (IS) jihadist group spearheading the offensive, but analysts say anger over perceived mistreatment by the authorities means they are less likely to cooperate with the security forces.

- Farcical parliament session -

Maliki's announcement came a day after an eagerly awaited opening to the Council of Representatives descended into chaos and ended in disarray as lawmakers traded heckles and threats.

So many Sunni and Kurdish deputies stayed away after a break meant to soothe soaring tempers that the quorum was lost and a speaker could not be elected, as constitutionally required.

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Washington quickly warned that "time is not on Iraq's side", with State Department spokeswoman Marie Harf calling for "extreme urgency".

UN special envoy Nickolay Mladenov said Iraqi politicians "need to realise that it is no longer business as usual".

Under a de facto agreement, the premier is a Shiite Arab, the speaker Sunni Arab and the president a Kurd.

Presiding MP Mahdi Hafez said the legislature would reconvene on July 8 if leaders were able to agree on senior posts.

In another sign of domestic political discord, Maliki on Wednesday rejected an assertion by the autonomous Kurdish region that its control of disputed territory is here to stay.

On the ground, Iraqi forces were struggling to break a stalemate with militants after initially wilting before the militant onslaught.

They have since performed more capably, albeit with limited offensive success.

However, the cost has been high. Nearly 900 security personnel were among 2,400 people killed in June, the highest figure in years, according to the United Nations.

Thousands of troops, backed by tanks, artillery and aerial cover, have made limited progress in retaking Tikrit which fell on June 11, with a highly publicised operation appearing to have hit difficulties.

- 'Advancing slowly' -

"They are advancing slowly because all of the houses and burned vehicles (en route to the city) have been rigged with explosives, and militants have deployed lots of roadside bombs and car bombs on the side of the roads," said Ahmed Abdullah Juburi, governor of Salaheddin province of which Tikrit is the capital.

Juburi said it would be days before security forces could make a concerted push into the city.

Maliki's security spokesman also told reporters that government forces had clashed with militants south of Baghdad.

In an effort to break the stand-off, meanwhile, the government bought more than a dozen Sukhoi warplanes from Russia.

Iraq has said it aims to begin using them for combat operations on Wednesday, but it was unclear if that has happened.

Loyalists are battling militants led by the IS, which Sunday declared a "caliphate", an Islamic form of government last seen under the Ottoman Empire, and ordered Muslims worldwide to pledge allegiance to their chief.

The announcement is an indicator of IS confidence, with its leader Abu Bakr al-Baghdadi even calling Tuesday for skilled professionals to join the cause, and marks a move against Al-Qaeda, from which the group broke away.

Also Tuesday, the Pentagon said that the nearly 500 US troops sent to Baghdad to bolster security are equipped with Apache attack helicopters and small unarmed surveillance drones.