The Iraqi parliament voted down Prime Minister Haidar al-Abadi's nominees for defence and interior minister Tuesday, leaving the key positions unfilled as the country battles to regain ground from militants.

Parliament approved Abadi's partial cabinet on August 8, ushering in his term as premier, but he asked for a week to present nominees for several ministries, including the security posts.

MPs rejected his candidates for the interior, defence and tourism ministers Tuesday, with some applauding after the no votes, though they did approve Mehsen Hassun as minister of water resources.

"I will consult with the (political) blocs and present the names when they are ready," Abadi said at the end of the session.

Iraq's last government also began without defence and interior ministers and the positions remained unfilled for the next four years, but Abadi said Tuesday that he wants to end the tradition.

"I am not ready... to administer these two ministries," he said, adding that he did so during the past week with "great difficulty".

Parliament speaker Salim al-Juburi announced that MPs would reconvene Thursday.

The security posts are empty at a time of crisis for Iraq, after militants led by the Islamic State (IS) jihadist group launched a major offensive in June, overrunning second city Mosul and much of the country's Sunni Arab heartland.

Washington called on Iraqi leaders to complete the formation of the government as soon as possible.

"We do appreciate the effort that Iraq's leaders have put forth thus far in forming an inclusive government," said State Department deputy spokeswoman Marie Harf.

But she said they "must act without delay and make the necessary decisions to complete the cabinet," adding the two posts were "a crucial part" of the national security plan.

The militants launched a renewed push in northern Iraq last month, driving Kurdish forces back towards their regional capital Arbil and sparking a campaign of American air strikes.

While initially restricted to the north, the US has carried out limited strikes elsewhere, including one in support of Iraqi security forces southwest of Baghdad.

IS also holds significant territory in neighbouring Syria, and has carried out beheadings and other atrocities in both countries that have shocked the world and galvanised international efforts against it.