UN Secretary-General Ban Ki-moon called on Monday for Iraqi leaders to address the "root causes" of a surge in bloodshed as security forces clashed with gunmen in violence-wracked Anbar province.

The UN chief's visit comes just months ahead of general elections but with the country hit by its worst spate of unrest since 2008 and an entire city and parts of another on Baghdad's doorstep in the hands of militants.

It is the first time fighters have exercised such open control in major cities since the insurgency that followed the 2003 US-led invasion, and Ban's remarks echoed US calls for Iraqi officials to focus on political reconciliation, in addition to ongoing military operations.

"I would urge the leaders of the country ... to address the root causes of the problems," Ban said during a joint news conference with Prime Minister Nuri al-Maliki.

"They should ensure that there is nobody left behind. There should be political cohesion" and "social cohesion, and political dialogue, inclusive dialogue," he said.

"The security situation in Iraq is undoubtedly a source of great concern," the UN chief said, adding that he is "deeply concerned by this escalation of violence in Anbar governorate."

Ban noted that civilian casualties were at the highest level since 2008, and said that "the government and people of Iraq must unite in addressing this terrorism."

He spoke after landing in the Iraqi capital for a two-day visit to the country, where he was also due to meet with parliament speaker Osama al-Nujaifi and other lawmakers, Vice-President Khudayr al-Khuzaie and the head of Iraq's election commission ahead of polls due in April.

Ban's visit comes as Iraq is embroiled in a bloody standoff between government forces and militants and anti-government tribes in Anbar, the mostly-Sunni desert province in west Iraq which shares a long border with Syria.

The country is also experiencing its wost prolonged period of violence since 2008, when it was emerging from a bloody Sunni-Shiite sectarian war that left tens of thousands dead.

In Anbar on Monday, clashes erupted between police and militants in Humairah, an area in the south of provincial capital Ramadi, when security forces attempted to reopen a police station in the neighbourhood, an AFP journalist at the scene said.

It was unclear if the firefight resulted in any casualties.

Fighting was also still raging in the Albubali and Khaldiyah areas between Ramadi and Fallujah, officials said.

Authorities meanwhile reopened a stretch of the main Iraq-Jordan highway that had been closed for months by Sunni protesters demonstrating against the alleged mistreatment of their community at the hands of the Shiite-led government.

Militants and anti-government tribes still hold two neighbourhoods in Ramadi, as well as all of Fallujah, a former insurgent stronghold just 60 kilometres (35 miles) from Baghdad.

Clashes still erupt periodically in the two cities but civil servants have returned to work in Ramadi and many residents who had fled Fallujah have since gone back.

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

At the same time, security forces have recruited their own tribal allies.

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

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, while the UN special envoy to Iraq has warned of a dire humanitarian situation.

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

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