Islamic State (IS) fighters scored a major victory in Iraq on Sunday, driving the last remaining government forces from Ramadi and decisively capturing the provincial capital of Anbar province, despite a recent spate of US airstrikes intended to keep the city from falling completely into militant hands.

IS fighters ransacked the local military headquarters, seized a large weapons cache, and executed several government loyalists, according to the New York Times, which cited information from security officials and tribal leaders.

"The city has fallen," Muhannad Haimour, a spokesman for Anbar's governor, reportedly said.

Video footage purportedly filmed on Sunday near Ramadi - which is located about 80 miles west of Baghdad - appeared to show dozens of vehicles, including several military Humvees and armored personnel carriers, speeding out of town.

Multiple security sources told McClatchyDC that IS fighters defeated reinforcements sent by the Iraqi government on Saturday to retake the besieged city. Several elite Iraqi units reportedly abandoned their US-provided equipment as they fled, the US news agency reported.

On Friday, IS militants set off a series of coordinated car bomb attacks to seize control of a provincial government compound housing the police station and governor's office. Video of the assault released by Aamaq News, a pro-IS media outlet, showed militants firing heavy machine guns from rooftops and black clouds of smoke wafting through deserted streets.

Haider Kata, an Iraqi reporter, told VICE News that tens of thousands of displaced people from the fighting are stuck in Anbar. Sponsorship from someone living inside Baghdad is necessary for families to leave and Anbar and travel there.

Government security forces that managed to escape the onslaught withdrew to Habbaniya, a massive military base about 20 miles southeast of Ramadi. An Iraqi television station reported they had created sand berms as a defensive measure at the base in hope of preventing IS from advancing further east.

In March, VICE News spoke with Iraqi officials who said that IS fighters would overrun the remaining portions of Ramadi under government control if the US-led coalition failed to provide adequate air support. Warplanes from the US-led coalition conducted at least seven airstrikes on Saturday and Sunday on targets in and around Ramadi.

Police officials in Ramadi previously told VICE News that more than 2,000 officers have been killed in Anbar since January 2014, when IS militants first arrived in the provincial capital. Anbar's largest hospital recorded more than 600 civilian deaths in the same period. Haimour, the spokesman for Anbar's governor, said at least 500 civilians and security personnel were killed over the last two days in and around Ramadi, the New York Times reported.

Anbar's Provincial Council reportedly met in Baghdad on Sunday and voted to ask Prime Minister Haider al-Abadi to send Shiite militias to rescue Anbar, Iraq's largest province, which has a majority Sunni population. Abadi subsequently issued a statement calling for the militias, including several backed by Iran, to be ready to fight in Anbar. The US has opposed deploying Shiite militias to Anbar over concerns that their presence could heighten sectarian tensions, according to the New York Times.