US Secretary of State John Kerry on Monday pledged "intense" support for Iraq against the "existential threat" of a major militant offensive pushing toward Baghdad from the north and west.

Kerry's surprise visit came as Sunni insurgents led by the jihadist Islamic State of Iraq and the Levant, or ISIL, seized a strategic town in northern Iraq, while security forces retook a border crossing with Syria.

The militant advance has not only put Prime Minister Nuri al-Maliki under pressure, but displaced hundreds of thousands and threatened to tear the country apart.

Kerry met with Maliki and other Iraqi leaders to urge a speeding up of the government formation process following April elections in order to 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 at the US embassy in Baghdad's heavily-fortified Green Zone.

"This is a critical moment for Iraq's future," Kerry said.

"It is a moment of decision for Iraq's leaders, and it's a moment of great urgency. Iraq faces an existential threat, and Iraq's leaders have to meet that threat with the incredible urgency that it demands."

Maliki also emphasised the danger of the crisis, telling Kerry it "represents a threat not only to Iraq but to regional and international peace."

- 'Hundreds' of soldiers killed -

Iraqi security forces are struggling to hold their ground in the face of militants who have seized major areas of five provinces.

Maliki's security spokesman, Lieutenant General Qassem Atta, said "hundreds" of soldiers were killed since the offensive began two weeks ago -- the most specific information by the government so far on losses in security ranks.

Iraqi security forces regained control of the Al-Waleed border crossing with Syria on Monday after militants withdrew, officers said -- a rare bright spot amid a series of recent setbacks.

But insurgents were able to overrun the strategic Shiite-majority northern town of Tal Afar and its airport after days of heavy fighting, an official and witnesses said Monday.

Atta said security forces were still fighting in the Tal Afar area, but added: "Even if we withdrew from Tal Afar or any other area, this does not mean that it is a defeat."

The town, located along a strategic corridor to Syria, had been the largest in the northern province of Nineveh not to fall to militants.

At the weekend, insurgents swept into the towns of Rawa and Ana in Anbar province, west of Baghdad, after taking the Al-Qaim border crossing with Syria.

The government said it made a "tactical" withdrawal from the towns, control of which allows the militants to widen a strategic route to Syria where they also hold stretches of territory.

- Convoy attack -

As Kerry began his visit, 69 detainees were killed in an attack by militants on a convoy carrying them in Babil province.

One policeman and eight gunmen were also killed in clashes that erupted during the attack in the Hashimiyah area, according to a police captain and a doctor.

Elsewhere, a family of six was killed on Baghdad's northern outskirts, while five Kurdish security forces members died in a bombing in northern Iraq.

ISIL aims to create an Islamic state incorporating both Iraq and Syria, where the group has become a major force in the rebellion against President Bashar al-Assad.

Washington wants Arab states to bring pressure on Iraq's leaders to speed up government formation, which has made little headway since April elections, and has tried to convince them ISIL poses as much of a threat to them as to Iraq.

Kerry warned all countries, particularly in the Gulf, that "there is no safety margin whatsoever in funding a group like ISIL."

The group has commandeered an enormous quantity of cash and resources as a result of the advance, bolstering coffers that were already the envy of militant groups around the world.

US leaders have stopped short of calling for Maliki to step down, but there is little doubt that they feel he has squandered the opportunity to rebuild Iraq since US troops withdrew in 2011.

"They must effect a unity that rises above the traditional divisions that have torn the government apart," Kerry said in Baghdad.

US President Barack Obama has offered to send up to 300 military advisers to Iraq, but has so far not backed air strikes as requested by Baghdad.

The White House said Monday that Iraq has offered legal guarantees to shield US special forces operatives sent to the country as advisers, in a diplomatic note to Washington.