Washington conceded Wednesday it is taking a "hard look" at its Iraq strategy after the fall of Ramadi, sending in anti-tank weapons to battle car bombs and working to shore up dispirited Iraqi forces.

After days of denying that the weekend capture of the Iraqi town by the Islamic State (IS) group was forcing a policy rethink, a top US official said since Sunday's "significant setback" the US had been focused solely on winning back Ramadi.

"You'd have to be delusional not to take something like this and say 'what went wrong, how do you fix it and how do we correct course to go from here?'," the official told a few foreign policy reporters.

"And that's exactly what we're doing. Taking an extremely hard look at it."

The Islamic State group's seizure of Ramadi, the first major city to fall to IS since the US and its coalition partners launched air strikes last August, was a painful blow to the US-led war against the jihadists.

It raised fresh doubts about Washington's war strategy, with analysts lining up to dub the town's capture a major setback.

- Ripple effect -

"We don't really have a strategy at all. We're basically playing this day by day," former defense secretary Robert Gates told MSNBC on Tuesday.

"Right now, it looks like they're (Iraq) going the way of Yugoslavia," he added.

Writing in the online Foreign Policy magazine, Middle East expert Hassan Hassan warned the fall of the city "marks a dangerous new phase of the war" saying it would have a "ripple effect across both the Syrian and Iraqi battlefields."

The IS group poses "a formidable and enormous threat," the State Department official agreed, adding already the jihadists were trying to use Ramadi's capture for propaganda purposes.

But he vowed: "In terms of taking back Ramadi, we're going to help the Iraqis do it as soon as possible."

He declined to give a specific time frame, but pledged there was 24-hour coalition air support over the town with a wave of fresh air strikes carried out Wednesday.

"When we see them in the streets of Ramadi, we're going to kill them in Ramadi."

IS has captured a swathe of territory in Iraq and Syria since early last year in its bid to set up an Islamic caliphate, and "sees this war as a war of expanding flags" he said, referring to the black flags the groups erects over captured territory.

- Gigantic blasts -

Asking not to be identified, the official said one of the most dangerous IS tactics was using huge "vehicle-borne improvised explosive devices" (VBIEDs) to plow into buildings and walls.

In Ramadi, a bulldozer packed with explosives was used to blow up the security perimeter around a central compound still held by government forces.

A total of 30 vehicles such as Humvees then flowed in, 10 of which were each carrying enough bomb-making materials to carry out explosions the size of the blast of the 1995 Oklahoma City bombing.

There were "gigantic explosions that took out entire city blocks," the official said.

"These enormous suicide VBIEDs is something that we have to help the Iraqis, and our partners in Syria, defeat."

During his key visit last month, Iraqi Prime Minister Haider al-Abadi asked the US administration for weapons systems to help counter the deadly car bombings.

"We made the decision immediately while he was here to get 1,000 AT4 anti-tank systems to Iraqi security forces and those are going to be arriving fairly soon," the senior State Department official told reporters.

The US administration has also already streamlined the process for delivering weapons to the forces on the ground, putting in place a system a few weeks ago with "an approved list" for small weapons with some provincial governors like in Anbar given authority to procure arms on their own without going through Baghdad.

Iraq forces eye swift Ramadi fightback before IS digs in
Baghdad (AFP) May 20, 2015 - Iraq's army and allied paramilitary forces massed around Ramadi on Tuesday, looking for swift action to recapture the city from the Islamic State group before it builds up defences.

With his security strategy in tatters and his authority facing its biggest challenge since he took office eight months ago, Prime Minister Haider al-Abadi was looking for quick redemption.

But the jihadists, whose capture of Ramadi on Sunday showed they may have been written off too soon, tried to keep up the momentum by attacking government and allied forces east of the city.

In the United States, President Barack Obama gathered national security advisors to weigh accelerated training and weapons supplies for Iraqi tribes and supporting an Iraqi-led counteroffensive.

"We are looking at how best to support local ground forces in Anbar" province, National Security Council spokesman Alistair Baskey told AFP, "including accelerating the training and equipping of local tribes and supporting an Iraqi-led operation to retake Ramadi".

Abadi has "ordered the setting up of new defence lines in Ramadi, to reorganise and deploy the fighting troops", his office said after talks with Iran's defence minister.

Reeling from the worst setback since IS grabbed swathes of territory in June last year, Abadi called in the Shiite-dominated Popular Mobilisation units (Hashed al-Shaabi).

He and Washington had hoped to rely on regular forces and locally recruited Sunni tribal fighters newly incorporated into the Hashed al-Shaabi.

Such a solution was seen as more palatable to the population of Anbar, a predominantly Sunni province, and a way for Washington to keep Iranian-backed militias at bay.

The Shiite paramilitary groups had been eager to join the Ramadi battle for some time and argued that Abadi's reluctance led to the provincial capital's fall.

- More planning needed -

Following a belated green light, they started sending convoys of fighters to Anbar, where anti-IS forces are massing, mostly east and west of Ramadi.

"The US government and Iraqi government seem to be on the same sheet of music that Ramadi has to be counterattacked before IS consolidates," Michael Knights of the Washington Institute said.

US Secretary of State John Kerry said on Monday he was "absolutely confident" the situation could be reversed within days.

In the other half of IS' self-proclaimed "caliphate", a monitoring group said US-led air strikes in the northeastern Syrian region of Hasakeh had killed 170 IS militants in 48 hours.

Anbar police chief Kadhim al-Fahdawi said a large number of well-prepared troops were positioned in Husaybah, about seven kilometres (less than five miles) east of Ramadi.

"This area will be the starting point for the operations to liberate the cities of Anbar," he said.

But much planning remains to be done before Iraqi forces attempt to move back into Ramadi, a large town on the Euphrates about 100 kilometres west of Baghdad.

"The military operation to liberate Ramadi and Anbar will not start until all the requirements are met," Fahdawi said.

Analysts say Washington's strategy of carrying out air strikes while the Iraqi military is trained up has shown its limits.

After holding on for a year and a half during which the jihadists never managed to take full control of the city, Iraqi security forces pulled out of their last bases on Sunday.

The retreat was chaotic, with groups of fighters leaving without command approval and others left stranded by their comrades.

At least 28 of them were plucked to safety in a dramatic helicopter rescue, footage of which has been aired on state TV, but many were killed and more are still missing.

The withdrawal also raised the question of the effective control the command had over its men and Iraq's cabinet on Tuesday suggested the defence of Ramadi had been less than heroic and could have been avoided.

In its cabinet meeting decisions, it supported "honouring the fighters who resisted the terrorist attacks and imposing the toughest punishment on the recalcitrants, whose attitude had consequences for Ramadi".

- 40,000 displaced -

According to an official in the Anbar governor's office, at least 500 fighters and civilians were killed in the three-day blitz leading to Ramadi's fall.

IS released pictures of the spoils they retrieved from abandoned government bases, including tanks, armoured personnel carriers and other vehicles, as well as weapons and ammunition.

The group also released pictures of the moment when its fighters freed prisoners held at a counterterrorism detention facility.

According to the International Organisation for Migration, fighting has forced 40,000 people from their homes, the second time in a month Ramadi residents had to flee.

"Thousands of people had to sleep in the open because they didn't have places to stay," said the UN's humanitarian coordinator, Lise Grande.

Officials said five displaced people, including two children, died Tuesday at the Bzeibez bridge where thousands have been waiting to cross into Baghdad governorate.