Iraqi security forces defend their headquarters against attacks by Islamic State extremists in the eastern part of Ramadi in Anbar province, May 14, 2015. Islamic State militants raised their black flag over the provincial government compound in the city of Ramadi in western Iraq on Friday, a Reuters witness said. The insurgents attacked Ramadi overnight using six suicide car bombs to reach the city centre, where the Anbar governorate compound is located, police sources said. Picture taken May 14, 2015. [Photo/Agencies]

BAGHDAD - Islamic State militants raised their black flag over the local government headquarters in the Iraqi city of Ramadi on Friday and claimed victory through mosque loudspeakers after overrunning most of the western provincial capital.

If Ramadi were to fall it would be the first major city seized by the insurgents in Iraq since security forces and paramilitary groups began pushing them back last year.

The insurgents attacked Ramadi overnight using six suicide car bombs to reach the city centre, where the Anbar provincial government compound is located, police sources said.

Fighting continued in one district of Ramadi, 100 km (60 miles) west of Baghdad, and government forces were still in control of a military command centre to the west of the city.

"The situation in Ramadi is dire, but the city has not fallen and the battle against criminal Daesh is still ongoing," Anbar governor Sohaib al-Rawi said on Twitter, using an Arabic name for Islamic State.

Prime Minister Haider al-Abadi met with military and security leaders on Friday as well as the heads of the air force and counterterrorism and pledged to intensify efforts to "expel the terrorist gangs from Ramadi".

He later spoke about the Ramadi attacks with U.S. Vice President Joe Biden, who pledged expedited U.S. aid to Iraqi forces including heavy weaponry, AT-4 shoulder-held rockets, ammunition and other supplies, the White House said.

Ramadi has been fought over for months, but the insurgents renewed their offensive there in April, crushing government forces' efforts to retake Anbar, Iraq's largest province, and move north to the Islamic State stronghold of Mosul.

An army major whose regiment is positioned near the Anbar operations command described the situation as critical and said the militants had taken control of the only major supply route into the city, making it difficult to send reinforcements.

Most army and police units have retreated to the area around the operation command to protect it, he said, but some elite counterterrorism forces were "fighting for their lives" in the Malaab district of central Ramadi, where they were surrounded.

"If the government does not send any reinforcements and the coalition air force does not rescue us, we will lose all of Ramadi by midnight," the major said.

"A massacre will take place and all of us will be slaughtered. We have been defending the city for months and we don't deserve to end like this. It's humiliating."

Still, the U.S. military sought to play down the Islamic State gains, telling a Pentagon news conference the militants were broadly "on the defensive" in Iraq.

Marine Corps Brigadier General Thomas Weidley, chief of staff for U.S.-led coalition operations, said Iraqi forces still controlled most "key facilities, infrastructure and lines of communication" in the Ramadi area.

"(Iraqi forces) will eventually take back the terrain," Weidley said via teleconference.