Secretary of State John Kerry on Monday, June 23 promised "intense and sustained" U.S. support for Iraq, but said the divided country would only survive if its leaders took urgent steps to bring it together, Reuters reported.

Hours before Kerry arrived in Baghdad, Sunni tribes who have joined a militant takeover of northern Iraq seized the only legal crossing point with Jordan, security sources said, leaving troops with no presence along the entire western frontier which includes some of the Middle East's most important trade routes.

U.S. President Barack Obama has offered up to 300 American advisers to Iraq but held off granting a request by Prime Minister Nuri al-Maliki's Shi'ite Muslim-led government for air strikes to counter the two-week advance by Sunni militants.

Officials have meanwhile called for Iraqis to form an inclusive government. The insurgency has been fuelled largely by a sense of materialization and persecution among Iraq's Sunnis.

"The support will be intense and sustained and if Iraq's leaders take the necessary steps to bring the country together, it will be effective," Kerry told reporters in Baghdad.

He said Maliki had "on multiple occasions affirmed his commitment to July 1" as the date to start the formation of a new government bringing in more Sunnis and Kurds to share power, a move Washington is keen to see.

Iraqi and Jordanian security sources said tribal leaders were negotiating to hand the Turabil desert border post to Sunni Islamists from the Islamic State in Iraq and the Levant (ISIL) who took two main crossings with Syria in recent days and have pushed Iraqi government forces back toward Baghdad.

Iraq state television said late on Monday that the army had recaptured both the crossing with Jordan and the al-Waleed crossing with Syria. Reuters could not independently confirm reports due to security restrictions.

Ethnic Kurdish forces control a third border post with Syria in the north, leaving government troops with no presence along Iraq's 800-km (500-mile) western border.

For the insurgents, capturing the frontier is a dramatic step towards the goal of erasing the modern border altogether and building a caliphate across swaths of Syria and Iraq.

Kerry said: "Iraq faces an existential threat and Iraq's leaders have to beat that threat with the incredible urgency that it demands. The very future of Iraq depends on choices that will be made in the next days and weeks."

Washington, which withdrew its troops from Iraq in 2011 after an occupation that followed the 2003 invasion that toppled dictator Saddam Hussein, has been struggling to help Maliki's administration contain a Sunni insurgency led by ISIL, an al Qaeda offshoot which seized northern cities this month.