Iraqi forces have launched a counteroffensive to retake areas of Anbar province recently captured by fighters from the Islamic State of Iraq and the Levant (ISIL) group.
Officials in Anbar told Associated Press news agency on Tuesday of air strikes and fighting on the ground west and south of the provincial capital, Ramadi.
A Shia unit spokesman told AFP news agency that Iraqi forces had surrounded Ramadi from three sides.
Thousands of Iraqi forces, made up of government troops and fighters from allied Shia militias under the Popular Mobilisation Forces banner, have been gathering at the Habbaniyah military base in preparation for the counteroffensive since Ramadi fell on May 17.
| Can Iraq win back Anbar province?
Al Jazeera's Imran Khan, reporting from Baghdad, said the fight for Ramadi was "likely to be a very protracted battle", adding that control over the major highways in Anbar would be key.
"It is quite likely to be a very big operation, mounted in stages along some of the main roads in Anbar province," he said.
Since the fall of Ramadi, Iraq and the US have traded accusations over who was to blame for the failure to maintain control of the provincial capital, which had been among just a few towns and cities to remain under government control in mainly Sunni Anbar.
"The Iraqis have got a very big point to prove that they can retake this province," our correspondent said.
Many tribal leaders in Anbar have voiced concerns that the inclusion of Shia units in the fight could cause sectarian divisions in the province.
Tens of thousands of residents from Ramadi have fled the city in recent weeks, causing a humanitarian crisis.
Source: Al Jazeera and agencies