The Iraqi military has launched a major operation on Ramadi city in the country's west to expel armed groups linked to al-Qaeda and end their weeks-long partial control of parts of the city.

Sunday's operation, which involved police, pro-government fighters and special forces, sought to wrest back key neighbourhoods of Ramadi, the capital of Anbar province and one of two cities where the authorities lost vast swaths of territory about three weeks ago.

Troops backed by tribesmen moved into five Ramadi neighbourhoods, with helicopters providing cover and firing on the district of Malaab at the centre of fighting between anti-government fighters and security forces and their tribal allies.

"The Iraqi army launched a large operation with helicopter cover against the Islamic State of Iraq and the Levant (ISIL), al-Qaeda and terrorists in Ramadi," Defence Ministry spokesman Staff Lieutenant General Mohammed al-Askari said, according to state television, employing terms frequently used by the Iraqi security forces to describe anti-government fighters.

The army's intervention, which has been resisted by Sunni anti-government tribesmen, comes after a pact was sealed on January 8 entailing that tribes will fight alongside local police against the al-Qaeda-linked ISIL.

The ISIL and its local allies overran parts of Ramadi, as well as the nearby city of Fallujah, on January 1 after security forces broke up a Sunni protest camp near Ramadi and arrested an outspoken Sunni lawmaker.

'Diabolical countries'

Army units have deployed around Fallujah, but Prime Minister Nouri al-Maliki has ruled out an all-out assault for now, saying the city's tribes must evict al-Qaeda fighters themselves. 

Maliki on Sunday blamed  "diabolical" and "treacherous" Arab countries for a protracted surge in nationwide violence, but stopped short of naming individual countries.

"Iraq is the target for some countries that are backing terrorism, and backing evil," Maliki said in a speech on Sunday in the southern city of Nasiriyah. 

"The world has united with us," he continued. "The [UN] Security Council, the European Union, and most Arab countries, except some diabolical treacherous countries."

Meanwhile, gunmen dressed in army uniforms killed at least six government-backed Sunni Muslim fighters at a checkpoint near the Iraqi city of Baquba, 65km northeast of Baghdad.

Iraq has witnessed a recent escalation of violence, especially with increased activity of the ISIL, which is also active in neighbouring Syria.

Source: Al Jazeera and agencies