Iraqi forces appear to be rallying and bolstering their defense of Baghdad in the face of Sunni extremists who have swept across the country's north, the Pentagon said Tuesday.

"We also have reason to believe -- certainly indications -- that the Iraqi security forces are stiffening their resistance and their defense and are coalescing, particularly in and around Baghdad, and that's encouraging," spokesman Rear Admiral John Kirby told reporters.

With Iraqi troops now receiving help from Shiite volunteers, Kirby said "it certainly appears as if they have the will to defend the capital."

The jihadist Islamic State of Iraq and the Levant (ISIL) have staged a stunning advance that has sent the US-trained Iraqi government army into a humiliating retreat, and now Baghdad itself is threatened.

After taking Mosul, the ISIL fighters captured a large area of mainly Sunni Arab territory stretching towards the capital.

The United States spent millions of dollars over several years training and arming a new Iraqi army after disbanding the Sunni-led force created by the late dictator Saddam Hussein.

Last week, Kirby acknowledged the Pentagon was "certainly disappointed in the performance of some of these units."

Iraqi soldiers and police in some cases fled en masse before the insurgent offensive, abandoning their vehicles and uniforms.

Iraqi Prime Minister Nuri al-Maliki sacked several senior officers over the recent defeats, and ordered one of them to face court-martial for desertion.

President Barack Obama has sent troops to Baghdad to strengthen security for the US embassy and is weighing potential military options, including air strikes, to counter the militant onslaught.

Obama has deployed about 170 forces to Baghdad and has another roughly 100 troops at the ready outside of Iraq for various "contingency" operations.

But Kirby said indications that Iraqi troops may be improving their defenses did not mean the White House would take more time to weigh potential military action.

He said that "everybody on the national security team understands the sense of urgency there in Iraq and the seriousness of the threat."

Iraq violence kills 21 as militants press offensive
Baghdad (AFP) June 17, 2014 - A series of bombings in Baghdad and shelling in another Iraqi city killed 21 people on Tuesday, while police found the bodies of 18 security personnel north of the capital.

The violence came during a major offensive, spearheaded by the jihadist Islamic State of Iraq and the Levant but involving other groups, which overran all of one province and chunks of three more in a matter of days.

In the deadliest single attack, a car bomb exploded in a market in the predominantly-Shiite Muslim area of north Baghdad, killing at least 11 people and wounding more than 20, security and medical officials said.

Five more bombings in the capital killed a further six people and wounded 14, officials said.

In Fallujah, a city west of Baghdad that has been held by anti-government fighters for more than five months, shelling killed four people and wounded three, Dr Ahmed Shami said.

North of the capital, Iraqi police discovered the bodies of 18 security force members who had suffered gunshots to the head and chest.

The bodies were discovered 15 kilometres (9 miles) east of the city of Samarra. It was unclear if they had been executed or died in fighting.

Iraqi security forces performed poorly during the initial days of the militant assault, in some cases abandoning uniforms, vehicles and positions to flee.

They seem to have recovered somewhat from the shock of the onslaught, retaking certain areas, but the militants have continued to gain ground elsewhere.