President Barack Obama on Monday pledged that Washington would remain a strong partner for Iraq as US troops exit by year-end, and played down the risk this departure creates a power vacuum Iran can exploit.

The withdrawal of almost all US troops from Iraq by December 31 has created uncertainty at a time the region remains roiled by the Arab Spring, and amid fear Syrian instability could spread sectarian strife into neighbouring Iraq.

But Obama told a press conference with Iraqi President Nuri Al-Maliki Washington remains a strong Middle East presence.   

"As we end this war and as Iraq faces its future, the Iraqi people must know that you will not stand alone. You have a strong and enduring partner in the United States of America."    

Fulfilling a vow to Americans weary of the nearly nine year old war as he campaigns for re-election in 2012, Obama's order for the troops to leave came after a deal to keep thousands of US trainers on the ground failed on the issue of immunity from prosecution in Iraq.

Almost 4,500 US troops have died since President George W Bush ordered the invasion in 2003, based on claims of weapons of mass destruction and al Qaeda ties that turned out not to exist.   

Obama and Maliki later visited Arlington National Cemetery for fallen American service members, and jointly laid a wreath on the Tomb of the Unknowns as the US and Iraqi national anthems were played.

Republicans criticise the president for not pushing harder to have some forces stay, amid concern about a power vacuum in the country that may be exploited by Iran, and as violence in neighboring Syria fans fear of regional sectarian strife.

The departure of US troops also raises questions about the ability of Iraqi security forces to keep the peace in a country still scarred by a bitter 2006-2007 civil war in which thousands died in sectarian and ethnic fighting.

Violence has diminished significantly since then but tension between Iraq's Shi'ite and Sunni Muslims and Kurds continues to restrain economic and political progress. Those divisions could be inflamed with no US forces left to play the role of intermediary.

Tehran has also sought to exert influence over Iraq's majority fellow Shi'ites since the United States toppled Iraqi dictator Saddam Hussein. But Maliki casts himself as a nationalist who will not bend to any outside power and Obama said he took him at his word: "I believe him. And he has shown himself to be willing to make very tough decisions in the interests of Iraqi nationalism, even if they cause problems with his neighbor."   - Reuters