Islamic State seizes Iraqi city stronghold

Islamic State militants have seized control of the main government building in the western Iraqi city of Ramadi, officials say.

IS militants have assumed control of most of the western Iraqi city of Ramadi, officials say. (AAP)

Islamic State militants have assumed control of most of the capital city of west Iraq's Anbar province, a security official says, marking a major advance for the extremist militia.

"Around 90 per cent of Ramadi is now under the control of Daesh," the official said, using an Arabic acronym for Islamic State.

Jihadists overran the provincial government's headquarters in the city centre after clashes with government troops. Ramadi is 110 kilometres west of the capital, Baghdad.

"Daesh elements also seized all weapons left behind by security forces." Later on Friday, jihadists blew up some buildings inside the governmental complex, according to witnesses.

Islamic State's advance into Ramadi deals a major setback to the efforts of the Iraqi government to drive the radical group out of the country.

The jihadists' latest gains have prompted the United States to send planes to transport American military experts from al-Habania air base, east of Ramadi, to Baghdad, a military Iraqi officer said.

Ramadi's fall to Islamic State will consolidate the militants' foothold in Anbar, Iraq's largest province, which stretches to Baghdad and the Syrian border.

Government troops, backed by allied militiamen and a US-led air campaign, started a large offensive last month to dislodge the extremists from Anbar. But the government action seems to have resulted in no significant success.

The hardliners' advances showed that Islamic State is still a formidable force, despite repeated attempts by the Iraqi government to rout the radical Sunni group.

Islamic State has controlled large areas of Sunni-dominated northern and western Iraq since the al-Qaeda splinter group launched a lightning offensive last year, during which government forces collapsed and fled.