US Secretary of State John Kerry is heading to the Middle East in a bid to unite Iraq's fractious leaders as Sunni militants seized more ground.
Shi'ite fighters paraded in Baghdad on Saturday in a dramatic show of force aimed at their Sunni opponents, who took control of a Syrian border crossing.
Washington's new diplomatic bid also aims to repel insurgents, whose lightning offensive has displaced hundreds of thousands and put Prime Minister Nuri al-Maliki under growing pressure.
Kerry flew to Amman on Saturday, and will then go to Brussels and Paris.
He is also expected to travel to Iraq, although details of the visit are not known.
Washington favoured Maliki when he became prime minister in 2006 as he was seen to crack down on Shi'ite militias while reaching out to Sunni leaders.
But he has recently made increasingly sectarian moves and is seen to have squandered the opportunity to rebuild since US troops withdrew in 2011.
"We gave Iraq the chance to have an inclusive democracy," President Barack Obama told CNN on Friday.
"Unfortunately, what we've seen is a breakdown of trust."
Obama has unveiled a plan to send 300 military advisors to Iraq, but has made it clear that without political change the US will not invest lives and resources.
Government security forces were holding on in several areas north of Baghdad, but officials said insurgents led by the Islamic State of Iraq and the Levant (ISIL) group had seized one of three official border crossings with Syria.
ISIL aims to create an Islamic state that will incorporate both Iraq and Syria, where the group has become a major force in the rebellion against President Bashar al-Assad.
The capture of the crossing came a day after 34 security force personnel were killed in the border town of Al-Qaim, leaving just one of three official crossings into Syria in the hands of the government; the third is controlled by Kurdish forces.
Anti-government fighters already hold parts of the western province of Anbar, which also abuts the Syrian border.
In a sign the broad alliance of ISIL jihadists and anti-government elements may be fracturing, internecine clashes killed 17 fighters in northern Kirkuk province.
The Sunni insurgents are made up of a broad alliance of other groups, including loyalists to former dictator Saddam Hussein.
In Baghdad, thousands of armed fighters loyal to powerful Shi'ite cleric Moqtada al-Sadr paraded, vowing to fight the offensive.
Similar parades were held in the Shi'ite heartland of southern Iraq.
UN aid agencies said they were rushing in supplies for more than one displaced million peopl
[You must be registered and logged in to see this link.]