Last week's sudden advance by ISIL - a group that declares all Shi'ites to be heretics deserving death and has proudly distributed footage of its fighters gunning down prisoners lying prone in mass graves - is a test for U.S. President Barack Obama, who pulled U.S. troops out of Iraq in 2011.
Obama has ruled out sending back ground troops but is considering other military options to help defend Baghdad, and U.S. officials have even spoken of cooperating with Tehran against the mutual enemy.
But U.S. and other international officials insist Maliki must do more to address the widespread sense of political exclusion among Sunnis, the minority that ran Iraq until U.S. troops deposed dictator Saddam Hussein after the 2003 invasion.
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U.S. Senate Majority Leader Harry Reid said he did not back sending U.S. troops into the conflict in Iraq, which he described as a "civil war", before a meeting with Obama about the crisis.
Reid and three other congressional leaders - Senate Republican leader Mitch McConnell, House of Representatives Speaker John Boehner and House Democratic Leader Nancy Pelosi - are meeting Obama later on Wednesday.
Western countries fear an ISIL-controlled mini-state in Syria and Iraq could become a haven for militants who could then stage attacks around the globe.
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British Prime Minister David Cameron told parliament he disagreed "with those people who think this is nothing to do with us and if they want to have some sort of extreme Islamist regime in the middle of Iraq it won't affect us. It will.
"The people in that regime, as well as trying to take territory, are also planning to attack us at home in the United Kingdom," Cameron said.
In a rerun of previous failed efforts at bridging sectarian and ethnic divisions, Shi'ite, Sunni and Kurdish leaders met late on Tuesday behind closed doors. They later stood frostily before cameras as Ibrahim al-Jaafari, a Shi'ite politician who held the post of prime minister before Maliki, read a statement.
"No terrorist powers represent any sect or religion," Jaafari said in the address, which included a broad promise of "reviewing the previous course" of Iraqi politics. Afterwards, most of the leaders, including Maliki and Usama al-Nujaifi, the leading Sunni present, walked away from each other in silence.
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Though the joint statement said only those directly employed by the Iraqi state should bear arms, thousands of Shi'ite militiamen have been mobilised to defend Baghdad.
According to one Shi'ite Islamist working in the government, well-trained organisations Asaib Ahl Haq, Khataeb Hezbollah and the Badr Organisation are now being deployed alongside Iraqi military units as the main combat force.
With battles now raging just an hour's drive north of the capital, Baghdad is on edge. The city of 7 million people saw fierce sectarian street fighting from 2006-2007 and is still divided into Sunni and Shi'ite districts, some protected by razor wire and concrete blast walls.
India said it was worried about 40 Indian construction workers missing in territory seized by ISIL.