WASHINGTON (AFP) - The United States hailed the nomination of a new Iraqi prime minister as a "key milestone" on Monday and offered to step up help in rolling back gains by Islamic militants.

Vice-President Joe Biden made the offer in a call to Iraqi President Fuad Masum, who had earlier asked Haidar al-Abadi to form a new government, replacing that of incumbent premier Nouri al-Maliki.

The Vice-President also relayed US President Barack Obama's congratulations and restated "his commitment to fully support a new and inclusive Iraqi government," the White House said.

According to the White House, Mr Abadi told Mr Biden he intended "to move expeditiously to form a broad-based, inclusive government capable of countering the threat of the Islamic State of Iraq and the Levant, and building a better future for Iraqis from all communities."

In his call to Mr Masum, Mr Biden commended the Iraqi President "for meeting this key milestone" of asking Mr Abadi to form a more inclusive new government.

Mr Biden "also emphasised President Obama's desire to boost coordination with a new Iraqi government and Iraqi security forces to roll back gains by the Islamic State of Iraq and the Levant."

Mr Masum acted over the objections of Mr Maliki, who gave a defiant televised speech at midnight suggesting he would fight to the end to keep his job.

The United States has expressed alarm over the rapid gains made by Islamic State militants since a lightning offensive in June that left Mosul and much of the Sunni heartland under the control of the extremists.

Last week, IS forces pushed into Kurdistan, spurring President Obama to order air strikes to protect Arbil, the capital, and call for a change in government in Baghdad to confront the threat.