Beleaguered Iraqi premier Nuri al-Maliki warned rivals Wednesday against exploiting a Sunni militant onslaught for political gain and insisted that any new government must be based on an election he won.

His remarks came as US military advisers began meeting with Iraqi commanders combatting an offensive that has overrun swathes of five provinces, killed nearly 1,100 people, displaced hundreds of thousands and threatens to tear the country apart.

And while security forces continued to repel assaults on critical towns and infrastructure, fighters from Al-Qaeda's Syrian franchise, Al-Nusra Front, made a local alliance with the jihadist group leading the charge in Iraq, bolstering its offensive.

In apparent response to calls from Sunni tribal leaders to form a government that ignores the result of an April 30 election, which they describe as a sham, Maliki said that would be a "coup against the constitution and the political process".

The incumbent premier, whose bloc won by far the most seats in April, said such a move was "an attempt by those who are against the constitution to eliminate the young democratic process and steal the votes of the voters."

He warned against exploiting "what the country is facing... in order to achieve political gains."

Though Washington has pressed for Iraq's fractious political leaders to unite in the face of the two-week campaign led by the Islamic State of Iraq and the Levant (ISIL) jihadist group, they have shown little sign of coming together.

- 'Intense, sustained' support -

US "support will be intense, sustained, and if Iraq's leaders take the necessary steps to bring the country together, it will be effective," Secretary of State John Kerry said during a visit to Iraq in which he met numerous politicians and urged them to work together.

However, US President Barack Obama has so far refrained from carrying out air strikes on the insurgents, as urged by Maliki.

Washington has stopped short of calling for Maliki to go, but there is little doubt it feels he has squandered the opportunity to rebuild Iraq since American troops withdrew in 2011.

Kerry is to hold meetings with Middle East allies in Paris Thursday to brief them on his talks in Iraq.

Maliki's security spokesman said first of up to 300 US military advisers had begun meeting with Iraqi commanders, adding: "We hope that there will be a true (US) intervention in order to offer real help for Iraq."

But the Pentagon spokesman Rear Admiral John Kirby said "this isn't about rushing to the rescue."

The advisers' primary task is to evaluate Baghdad's forces, with the Pentagon saying the US has expanded its surveillance flights over Iraq, conducting 30-35 sorties daily.

The advisers come as the Iraqi military is performing better after wilting in the initial stages of the onslaught, which began June 9.

Loyalists fought off insurgent attacks Wednesday on a major air base and a key western town, after repelling assaults on Iraq's biggest oil refinery.

Militants and security forces clashed periodically overnight, but government troops maintained control of the Balad air base, a tribal leader and a security official said.

Another offensive was repelled in Haditha in Anbar province, west of Baghdad.

Bombings and shelling south of Baghdad and in the disputed, ethnically mixed northern oil hub of Kirkuk killed 17 people.

Maliki's security spokesman says hundreds of soldiers have been killed since the offensive began.

- Dangers of food shortage -

In other developments, fighters from Al-Nusra Front in the town of Albu Kamal on the Syrian border with Iraq pledged loyalty to ISIL, giving it control over both sides of the frontier.

The move reflects how ISIL is fast gaining the upper hand in eastern Syria, where it has been locked in combat with fighters from the Al-Qaeda franchise and its allies virtually all year.

ISIL adheres to a harsh interpretation of Islamic law and considers Iraq's majority Shiite population to be heretics.

It aims to create an Islamic state straddling Iraq and Syria, where it has become a major force in the rebellion against President Bashar al-Assad.

It has commandeered an enormous quantity of cash and resources during the advance, boosting its coffers.

The United Nations says at least 1,075 people have been killed and 658 wounded in Iraq between June 5 and 22.

But the violence itself is not the only threat posed by the conflict.

The UN food agency warned that Iraq's harvest is under threat and the country is "facing serious food security concerns.

The world body also tripled its appeal for aid funding in Iraq this year to more than $312 million (229 million euros).