United States Vice President Joe Biden has said that "serious" and "committed" progress is being made towards defeating the Islamic State of Iraq and the Levant (ISIL) group in Iraq, but warned that the political crisis in the country threatened to undo those gains.

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On his first trip to Iraq since 2011 on Thursday, he met Iraq's beleaguered leaders and praised them for working "very, very hard" to construct a new cabinet and touted progress in taking back territory from ISIL.

Biden said he and Prime Minister Haider al-Abadi discussed "plans in store for Mosul" and said he was "very optimistic".

US officials have not put a timeline on reclaiming Mosul, the de facto Iraqi capital for ISIL, which is also known as ISIS, but said that they expect progress to slow during the summer.

"It's real, it's serious, and it's committed," Biden said as he met Parliament Speaker Salim al-Jabouri, a Sunni politician facing calls from his colleagues to resign.

Senior US officials travelling with Biden told the Associated Press news agency that any lost momentum would probably be due to political unrest rather than military shortcomings.

The officials, who requested anonymity, said the present infighting risked becoming a distraction, with politicians more focused on keeping their jobs than fighting ISIL.

Sadr's challenge to Iraq

The visit comes amid a wave of protests and demands for sweeping political reforms that have paralysed a government already struggling with a dire economic crisis and ISIL.

The turmoil engulfing Iraq's government grew out of weeks of rallies by followers of influential Shia leader Muqtada al-Sadr demanding an end to pervasive corruption and mismanagement.

Thousands have protested outside Baghdad's heavily guarded Green Zone, calling for politicians to be replaced by independent technocrats and for Iraq's powerful Shia militias to be brought into key ministries.

The turbulence has gripped the Iraqi capital. Last month, al-Abadi pulled troops fighting ISIL on the frontlines to protect Baghdad amid the protests.

The cost of the war against ISIL, along with the plunge in the price of oil - which accounts for 95 percent of Iraq's revenues - has caused a budget deficit which Iraqi officials predict will be more than $30bn this year.

Source: Agencies