ISIS militants raised their black flag over the local government compound in the Iraqi city of Ramadi on Friday after over-running most of the western provincial capital.
The insurgents detonated four suicide car bombs in the early morning and drove Iraqi security forces from defensive lines, a senior security official said.
The ISIS fighters who stormed the complex were armed with light and heavy weapons and with armored vehicles, the security official said.
The official said Friday night that reinforcement troops had arrived in Ramadi, 60 miles west of Baghdad, and fighting was ongoing between Iraqi security forces and ISIS inside Ramadi's government district.
According to the official, two districts just outside the center of the city, each belonging to the Iraqi security forces, still remain under government control, including the headquarters of the 8th brigade of the Iraqi Army.
One anti-ISIS fighter inside Ramadi who NBC News is not naming due to security concerns said Friday that ISIS blew up at least two buildings inside the government complex: an administrative building and a police building.
The fighter, a member of a Sunni tribal militia opposed to ISIS, estimated that 90% of Ramadi's central district is now held by ISIS. NBC News could not independently verify his claim.
Friday's events mark a grim turn in a weeks-long fight for the control of Ramadi.
In April, ISIS fighters seized territory north and east of Ramadi, spurring a refugee crisis. According to the UN's refugee agency, an estimated 114,000 Iraqis have fled Ramadi.
If Ramadi were to fall completely to ISIS it would be a strategic blow to Prime Minister Haider al-Abadi's government, barely six weeks after the army and Shiite militias recaptured the city of Tikrit from ISIS fighters.
The militant group said it had stormed Ramadi's provincial government compound and taken control after "eliminating the apostates" who remained inside.
Police said the militants used an armored bulldozer to remove blast walls blocking the road to the police department adjacent to the provincial government building and blew the vehicle up when it reached there.
A senior security official said Friday that Iraqi security forces withdrew from the center of the city after the dawn raid and after at least four ISIS suicide attacks. According to the official, the Iraqi central government later dispatched more troops to Ramadi, but it is unclear what effect the reinforcements had.
According to Charlie Winter, a researcher who monitors jihadist activity online for the London-based Quilliam foundation, prominent ISIS Twitter accounts Friday published updates on the advance.
Winter said one Arabic-language pro-ISIS Twitter account said the organization's flag now flies over the government center. "Ramadi hospital and its university are in the hands of the Islamic State," another account said.
Reuters also reported Friday that eyewitnesses saw the ISIS flag flew above Ramadi's government center.
Hospital sources said in Ramadi said at least 11 people had been killed in the attack.
The senior Iraqi security official told NBC News Friday that plans were in place to send more reinforcements, and that urgent meetings are being called in Baghdad.
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— James Novogrod and NBC News
Reuters contributed to this report.