The United States vowed Wednesday to boost aid to Iraq amid fears the US-backed Iraqi army is increasingly powerless against emboldened militants more than two years after American forces withdrew.

But State Department spokeswoman Jen Psaki denied the offensive by the Islamic State of Iraq and Levant had caught Washington by surprise or that it marked a failure of US policy in the country it invaded in 2003.

Washington is committed to "working with the Iraqi government and leaders across Iraq to support a unified approach against ISIL's continued aggression," Psaki told reporters, adding that the US administration had long warned of the dangers posed by the militants now sweeping toward Baghdad.

A US official said President Barack Obama's administration was considering sending "more weaponry" to Iraq after ISIL seized the cities of Mosul and Tikrit.

But there was no current plan to send US troops back into Iraq, where around 4,500 American soldiers died in the bitter conflict, Psaki said.

The United States has already expedited arms shipments to Iraq this year and ramped up training of Iraqi security forces, while Congress is mulling a request for a further $1 billion in military aid.

In January, Washington sold 24 Apache attack helicopters to Baghdad, as well as about 300 anti-tank Hellfire missiles and two of some 36 F-16 fighter aircraft, a Pentagon spokesman said. Some of the arms have been delivered and others should be on their way in the coming months.

The new $1 billion includes provisions for around 200 Humvee vehicles and 24 AT-6C Texan II aircraft, but it may take months to get lawmakers' approval.

The last US troops left Iraq in December 2011, eight years after ousting Saddam Hussein following the invasion ordered by then president George W. Bush more than a decade ago.

But the country has been left riven by deep sectarian divisions between Sunni and Shiites Muslims.

Psaki repeated calls for "a strong unified front" from Iraqi leaders, saying Washington was taking steps "to cooperate on a security plan that will enhance the Iraqi army's ability to hold positions and confront this ISIL aggression."

Since ISIL began its spectacular assault late Monday, militants have captured a large swathe of northern and north-central Iraq, causing about half a million residents to flee their homes in Mosul.

A majority Sunni Muslim city, Mosul has long felt marginalized by Prime Minister Nuri al-Maliki's Shiite-led government.

Both the United States and Iraqi leaders have come under fire for failing to prevent the growth of militant groups, many of them armed and trained amid the conflict in neighboring Syria.

- Iraq didn't ask US troops to stay -

US envoy to Jordan Stuart Jones, tapped to be the next ambassador to Baghdad, highlighted that Iraqi leaders had refused to sign a security pact with the Obama administration.

"The Iraqi people really did not come together and asked us to stay in a way that made possible for us to stay," Jones told US lawmakers.

Security expert Bruce Riedel told AFP that "there's plenty of room for finger-pointing for the debacle in Iraq."

"Let's not forget the disastrous decision to start the war in 2003 as the place to begin finger-pointing," the senior fellow at the Brookings Institution added.

Calling for renewed focus on the current situation, Riedel said the Pentagon needed to make a fundamental assessment of the problems facing the Iraqi military.

"Is it a lack of Apache helicopters, or is it a fundamental breakdown in the cohesion of the Iraqi military?" he asked.

"If... it's a problem that the Iraqi military is broken at its core, then there's no point in sending more Humvees and Apaches," Riedel added.

"It's a point of how do we minimize our losses and live with what might be rapidly be developing as a de-facto partition of Iraq between a Sunni extremist state and a Shiite state."