Iraqi government forces have launched an attempt to retake Anbar from Islamic State (IS) militants, and have partially encircled the provincial capital of Ramadi, an official said on Tuesday.

Ahmed al-Assadi, a member of parliament and spokesman for the Hashd al Shaabi Shia militia groups, also known as the Popular Mobilization Forces, announced an operation named "Labaik ya Hussein" on state TV. He added that it was not expected to "last for a long time" and that Ramadi was already surrounded on three sides, the Associated Press reported. Assadi also said new weapons which would "surprise the enemy" were being deployed by government forces and stated: "We believe that liberating Ramadi will not take long."

Assadi described an offensive designed to cut off IS in Ramadi conducted by security forces and allied militia fighters, which would close in from Salaheddin province to the north. "The operation's goal is to liberate those regions between Salaheddin and Anbar and try to isolate the province of Anbar," he told AFP.

IS controls much of Anbar and has done since early 2014. Government troops had repelled its repeated attempts take Ramadi, but the jihadists launched a large offensive earlier this month that routed security forces and eventually raised their black flag over the city on May 17. It was an embarrassing defeat after Iraqi forces backed by US-led air support had been making slow but steady progress in pushing IS back elsewhere in the country.

Also on Tuesday, a US military spokesperson said that 2,000 anti-tank rockets would be deployed to Iraq "within the next week" to help in the battle against IS.

Anbar is majority Sunni province, and Prime Minister Haider al-Abadi's government had seemingly avoided calling on Shia militias for help to avoid inflaming sectarian tensions.

Paramilitaries have played a major part in the Iraqi fight against IS, filling the gaps after the spectacular collapse of a significant part of the regular armed forces during the group's June 2014 offensive. In some cases, this growth in power and influence seems to have been accompanied by a return to the campaigns of kidnappings and killings last seen during the brutal sectarian violence that almost tore Iraq apart in 2006 and 2007. Rights groups have said the militias operate outside the law, with little or no accountability, and accused them of committing atrocities and war crimes.