Iraq's premier has rejected forming a unity government to confront jihadists whose sweeping offensive in the country has been bolstered when al-Qaeda's Syrian franchise pledged loyalty to them at a border town.
NATO is to hold key discussions on Wednesday on Iraq, where the UN says nearly 1100 people have been killed as Sunni militants led by the jihadist Islamic State of Iraq and the Levant (ISIL, also known as ISIS) overran swathes of five provinces north and west of Baghdad this month.
The onslaught has displaced hundreds of thousands, alarmed world leaders and put Iraq's Shiite prime minister, Nuri al-Maliki, under pressure at home and abroad.
The Syrian Observatory for Human Rights said the Al-Nusra Front, al-Qaeda's affiliate in Syria, pledged loyalty at the Iraq-Syria border to ISIL, a group it has battled for months.
The move clears the way for a joint push to take control of both sides of the frontier between eastern Syria and western Iraq, and removes a threat to ISIL.
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"They are rivals, but both groups are jihadist and extremists," said Observatory director Rami Abdel Rahman.
In a televised address, Maliki ruled out forming a national emergency government to confront the crisis.
"The call to form a national emergency government is a coup against the constitution and the political process," he said.
"It is an attempt by those who are against the constitution to eliminate the young democratic process and steal the votes of the voters," added Maliki, whose bloc won by far the most seats in April 30 elections but fell short of an outright majority.
His remarks came a day after the first of up to 300 US military advisers began their mission to help the Iraqi army, but the Pentagon said the Americans were not taking on a combat role.
Their primary task is to evaluate Iraqi forces and not to turn the tide against the militants, Pentagon spokesman Rear Admiral John Kirby said.
He added that the United States had expanded its surveillance flights over Iraq, with manned and unmanned aircraft, and was conducting 30-35 sorties daily.
Washington has said it has received legal guarantees from Iraq to shield the advisers, but they fall short of the parliament-approved legal immunity it demanded during talks on a post-2011 American military presence in the country.
US Secretary of State John Kerry huddled with European allies late Tuesday, ahead of the NATO talks in Brussels, after a whirlwind visit to Iraq aimed at shoring up unity in the country.
Kerry met with EU foreign policy chief Catherine Ashton as well as other European partners and "discussed the grave security situation in Iraq," a spokeswoman said.
"As everybody knows this is a very critical time for Iraq," Kerry had warned in the northern Iraqi city of Arbil, where he met with Kurdish leaders on Tuesday.
The militant offensive allowed Iraqi Kurds to take control of disputed territory they want to incorporate into their autonomous region over Baghdad's strong objections.
Security forces have held off recent assaults on Iraq's largest oil refinery and a strategic western town, but these successes were marred by air strikes that killed civilians.
