US Secretary of State John Kerry has urged Iraq's top Shiite leaders to give more government power to political opponents before a Sunni insurgency seizes more control across the country and sweeps away hopes of lasting peace.

The closed-door meeting in Baghdad between Mr Kerry and Iraqi PM Nouri al-Maliki was not thought to have been friendly, given that officials in Washington have floated suggestions the premier should resign as a necessary first step towards quelling the vicious uprising.

However, Mr Kerry appeared encouraged after the discussion with Mr al-Maliki, which ran for a little over 90 minutes.

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He pledged that US support would be "intense and sustained" to help them combat the Islamist insurgency that has swept through the country's north and west.

President Barack Obama offered up to 300 American advisers to help coordinate the fight. But he held off granting a request for air strikes from Maliki's Shi'ite Muslim-led government and renewed a call for Maliki to do more to overcome sectarian divisions that have alienated the Sunni Muslim minority.

"The key today was to get from each of the government leaders a clarity with respect to the road forward in terms of government formation," Mr Kerry said. "Indeed, Prime Minister Maliki firmly and on multiple occasions affirmed his commitment to July 1 (to form the new government)."

Mr Kerry said Obama will not wait before he acts to provide advisers and support for Iraq's military. "The support will be intense and sustained and if Iraq's leaders take the necessary steps to bring the country together, it will be effective.

"It will allow Iraqi security forces to confront ISIL more effectively and in a way that respects Iraq's sovereignty while also respecting America's and the region's vital interests," he said, referring to the Islamic State in Iraq and the Levant insurgents who have spearheaded the Sunni revolt.

Mr Kerry also met the influential Shiite cleric Ammar al-Hakim, and parliament speaker Osama al-Nujaifi and deputy prime minister Saleh al-Mutlaq - the nation's two highest-ranking Sunnis.

Iraqi officials briefed on Mr Kerry's talks said Mr al-Maliki urged America to target the militants' positions in Iraq and neighbouring Syria, citing training camps and convoys, with air strikes. The officials said Mr Kerry responded by saying a great deal of care and caution must be taken before attacks are launched to avoid civilian casualties that could create the impression Americans are attacking Sunnis.

President Obama said Mr al-Maliki and the Iraqi leadership faces a test as to whether "they are able to set aside their suspicions, their sectarian preferences for the good of the whole".

The president added: "The one thing I do know is that if they fail to do that then no amount of military action by the United States can hold that country together."

Mr Kerry's meeting came as Iraq's forces abandoned the border with Jordan, leaving the entire Western frontier outside government control.

For the insurgents, capturing the frontier is a dramatic step towards the goal of erasing the modern border altogether and building a caliphate across swathes of Syria and Iraq.