Clearing Fallujah and Ramadi of al-Qaeda by the US-trained and equipped Iraqi army won’t be easy as ISIS fighters are highly motivated, heavily armed and well trained.

BAGHDAD (AP) — Fierce clashes erupted Tuesday between Iraqi special forces and al-Qaeda-linked militants outside the city of Fallujah, a flare-up in a days-long standoff in the Sunni-dominated western province of Anbar, Iraqi officials said.

The fighting broke out about 12 miles west of Fallujah, following the capture of an army officer and four soldiers in the area a day earlier, Anbar provincial spokesman Dhari al-Rishawi told The Associated Press.

There was no immediate word on casualties. The clashes began just hours after deadly bomb attacks killed at least four people elsewhere in the country.

Iraqi security forces and allies from Sunni tribes have been battling militants to recapture two key cities in Iraq’s western Anbar province, Fallujah and Ramadi, the provincial capital. Fighters from an al-Qaeda-linked group, known as the Islamic State in Iraq and the Levant, control the center of Fallujah, 40 miles west of Baghdad, and part of the nearby Ramadi.

The al-Qaeda fighters’ seizure of the two cities — once bloody battlegrounds for U.S. troops — poses the most serious challenge to Prime Minister Nouri al-Maliki’s Shiite-led government since the departure of American forces in late 2011.

The immediate trigger for the unrest was the Dec. 28 arrest of a Sunni lawmaker sought on terrorism charges, followed by the government’s dismantling of a months-old anti-government Sunni protest camp in Ramadi.