Al-Qaeda's Syrian franchise formed a local alliance Wednesday with the powerful jihadists leading a major offensive in Iraq, as the first of up to 300 US military advisers moved to Baghdad.
NATO is to hold key discussions Wednesday on Iraq, where the UN says nearly 1,100 people have been killed as Sunni militants, led by the jihadist Islamic State of Iraq and the Levant (ISIL), overran swathes of five provinces north and west of Baghdad this month.
The onslaught has displaced hundreds of thousands of people, alarmed world leaders and put Prime Minister Nuri al-Maliki under pressure at home and abroad.
The Syrian Observatory for Human Rights monitoring group said the Al-Nusra Front, Al-Qaeda's affiliate in Syria, on Wednesday made a local pledge of loyalty at the Iraq-Syria border to ISIL, a group that it has battled for months.
The move clears the way for a joint push by the two groups to take control of both sides of the frontier between eastern Syria and western Iraq, and removes a threat to ISIL.
"They are rivals, but both groups are jihadist and extremists," said Observatory director Rami Abdel Rahman.
The first of up to 300 US military advisers began their mission in Baghdad Tuesday to help the Iraqi army, but the Pentagon said the American troops were not taking on a combat role.
The primary task of the advisers was to evaluate the state of the Iraqi forces and not to turn the tide against the militants, the Pentagon's press secretary said.
"This isn't about rushing to the rescue," Rear Admiral John Kirby said, adding however the US was ready to carry out bombing raids if called upon.
He said 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 warned in the northern Iraqi city of Arbil on Tuesday as he met with Kurdish leaders.
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.
Maliki's security spokesman said hundreds of soldiers have been killed since the offensive began on June 9 -- the most specific official information so far on government losses.
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.
The UN says at least 1,075 people have been killed, an estimated three-quarters of them civilians, and 658 wounded in Iraq between June 5 and 22.
Kerry has urged the speedy formation of a government in Iraq following April elections so the country can better face down the insurgents.
Washington's "support will be intense, sustained, and if Iraq's leaders take the necessary steps to bring the country together, it will be effective," Kerry said.
However, US President Barack Obama has so far refrained from carrying out air strikes on the insurgents as urged by Maliki.
US leaders have stopped short of calling for Maliki to go, but there is little doubt they feel he has squandered the opportunity to rebuild Iraq since American troops withdrew in 2011.
ISIL aims to create an Islamic state incorporating both Iraq and Syria, where it has become a major force in the rebellion against President Bashar al-Assad.
It has commandeered an enormous quantity of cash and resources during the advance, bolstering coffers already the envy of militant groups worldwide.
Kerry will Thursday hold back-to-back meetings with Gulf allies in Paris to brief them on his talks in Iraq and discuss the bloody three-year war in Syria.
He will meet Saudi Foreign Minister Saud al-Faisal, top United Arab Emirates diplomat Abdullah bin Zayed and his Jordanian counterpart Nasser Judeh, a senior State Department official said.