Sunni gunmen, including fighters linked to Al-Qaeda, made gains in the contested Iraqi city of Ramadi Tuesday in a setback for pro-government forces, as attacks killed 18 around the capital.

The clashes came after UN Secretary General Ban Ki-moon appealed to Iraqi leaders to address the "root causes" of nationwide unrest, echoing calls from diplomats for Baghdad to focus more on political reconciliation.

But Prime Minister Nuri al-Maliki has ruled out talks to resolve the standoff, as April 30 parliamentary elections loom and his government faces the worst protracted period of bloodshed since Iraq was emerging from a brutal Sunni-Shiite sectarian war in 2008.

Parts of Ramadi and all of Fallujah, just 60 kilometres (40 miles) from Baghdad, fell out of government control more than two weeks ago, the first time militants have exercised such open control in major cities since the height of the insurgency that followed the US-led invasion of 2003.

In recent days, Iraqi forces and allied tribes had been retaking areas of Ramadi, the Anbar provincial capital, from militants and anti-government tribesmen. The latest clashes represent a setback for Baghdad and threaten to further prolong the crisis.

Gunmen, including those affiliated with the Al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL), seized all or parts of a half-dozen neighbourhoods in the south and centre of the city in firefights that began late Monday evening and carried on into Tuesday, according to a police captain and an AFP journalist.

Two policemen were killed and five others wounded in the clashes, and three police vehicles set ablaze, according to Dr Ahmed al-Ani of the city's main hospital.

Sporadic clashes continued in the affected neighbourhoods Tuesday, while shelling struck the Andalus neighbourhood of central Ramadi and damaged houses, a police officer said.

Most civil servants had returned to work and many shops reopened, the AFP correspondent reported, but schools remained closed.

Gunfights also erupted in the Albubali area between Ramadi and Fallujah where security forces have repeatedly clashed with militants.

In Fallujah, government employees returned to work, but the city remained in the control of gunmen, an AFP journalist reported.

A doctor in the city's main hospital said at least 26 people were killed in Fallujah and surrounding areas over the past two weeks.

The army stayed on the city's eastern edge Tuesday. Shelling in the city wounded two people, witnesses said, while brief clashes could be heard in the city Monday evening.

Fighting erupted in the Ramadi area on December 30, when security forces cleared a year-old Sunni Arab anti-government protest camp.

The violence spread to Fallujah, and militants moved in and seized the city and parts of Ramadi after security forces withdrew.

'There is no dialogue with Al-Qaeda': Maliki

ISIL has been active in the Anbar fighting, but so have anti-government tribesmen.

The army has largely stayed outside of Fallujah during the crisis, with analysts warning any assault on the city would likely cause significant civilian casualties.

In Baghdad, shootings and bombings across the capital and a nearby town killed 18 people, including a senior judge.

Authorities also found the bullet-riddled body of a young woman a day after attacks in and around the capital, including four car bombs against civilian targets, killed 30 people.

UN chief Ban urged Iraqi leaders to address the root causes of the unrest, and "ensure that there is nobody left behind. There should be political cohesion" and "social cohesion, and political dialogue, inclusive dialogue."

Ban's remarks, at a news conference with Maliki on Monday, echoed US calls for officials to focus on political reconciliation in addition to ongoing military operations.

But Maliki insisted that "what is happening in Anbar has no relation to Iraqi problems," and ruled out dialogue with jihadists.

"Dialogue with whom -- with Al-Qaeda? There is no dialogue with Al-Qaeda, and the Iraqi national decision is to end Al-Qaeda," Maliki said, referring to ISIL.

The Iraqi Red Crescent said it had provided humanitarian assistance to more than 8,000 families across Anbar but that upwards of 13,000 had fled Fallujah.