The seizure by Al-Qaeda-linked militants of a major Iraqi city and parts of another illustrates their resurgence, and harkens back to the darkest days of the insurgency that followed the 2003 US-led invasion.

The Al-Qaeda franchise in Iraq fell from the height of its influence in the years after the invasion, suffering defeats by American forces, especially after Sunni tribesmen joined them from late 2006 in a process that became known as the "Awakening."

But it has made a striking comeback in its latest incarnation, the Islamic State of Iraq and the Levant (ISIL), which spans more than one country and has been bolstered by the cross-border ties it has established in Syria during the country's civil war, analysts say.

It is now a major force in the Syrian conflict, and has also carried out operations in Iraq ranging from brutal bombings of civilians to brazen prison assaults.

"ISIL has been able to leverage its networks and capabilities in Iraq to become a strong presence in Syria, and has used its presence in Syria to leverage its position in Iraq," said Daniel Byman, a senior fellow at the Brookings Institution's Saban Center for Middle East Policy.

"It now is again able to conduct limited guerrilla war as well as a sustained campaign of terrorism," he said.

But ISIL went far beyond its usual bombings and hit-and-run attacks when it seized parts of the Anbar province cities of Ramadi and Fallujah, west of Baghdad, which it has held for days.

A senior security official said Saturday that Fallujah was completely under ISIL control, with witnesses reporting ISIL militants in both cities, including fighters patrolling them in vehicles.

On Friday, hundreds of gunmen, some bearing the black flags often flown by jihadists, gathered at outdoor weekly Muslim prayers in Fallujah, where one militant announced that "Fallujah is an Islamic state."

Fighting began in the Ramadi area Monday -- when security forces broke up the country's main Sunni Arab anti-government protest camp -- and then spread to Fallujah.

Security withdrew from areas of both cities, which cleared the way for militants to move in.

More than 160 people have been killed in fighting between ISIL, security forces and tribesmen in just two days.

'Objectives far beyond Iraq'

ISIL's "strength and territorial control and influence has been expanding in Anbar for some time," said Charles Lister, a visiting fellow at the Brookings Doha Center.

Its "objectives lie far beyond Iraq, but transnational objectives of establishing an Islamic state across the Levant can only be realised once mini-states of territorial control are realised," Lister said.

"In the Iraqi context, Anbar and also Nineveh (province) are of crucial importance as a result of their direct links into eastern Syria."

Defence ministry spokesman Mohammed al-Askari has also highlighted the importance of the Syrian connection.

Aerial photographs and other information point to "the arrival of weapons and advanced equipment from Syria to the desert of western Anbar and the border of Nineveh province," encouraging militants to rebuild once-eliminated camps, Askari said.

Security forces have targeted militant camps in recent operations in western Iraq.

John Drake, a security specialist with risk management firm AKE Group, said the situation in Anbar "is comparable to the bad days at the height of the insurgency."

But while it may add to ISIL's credibility, attempting to hold territory poses risks.

"It will give more credibility to the group, but in the longer term, it will have to tread very warily if it is to avoid incurring the wrath of the local population again," Drake said, referring to disgust with militants' brutality that led tribal fighters to turn against them.

"ISIL showed that it can strike fast and hard, exploiting the gap as Iraqi army forces withdrew from urban areas to overrun police stations," said Michael Knights, a fellow at the Washington Institute for Near East Policy.

"But the growing strength of ISIL could hurt the organisation because fighting open battles for urban terrain plays to the government's strengths," he said.

The situation in Anbar presents both risk and opportunity, Knights said.

"If the Iraqi government uses this moment to re-engage the tribal awakening, Al-Qaeda could be dealt a severe reverse. If the government sidelines the Sunni tribes and continues with a brute force approach, Al-Qaeda may gain significantly in strength."