The terrorists used car bombs and mortars in the offensive targeting Ramadi, the main city of the mostly Sunni western province of Anbar, officials said.
"Daesh terrorists today mounted a violent attack on Ramadi from four directions," said chief of Anbar police, Kadhem al-Fahdawi.He gave no casualties, citing the intensity of fighting.
"Security forces, backed by tribal fighters and the air force, are trying to repulse the assault," he told Iraqi site Alsumaria News. "Clashes are still going on."
"IS" fighters pounded the city Centre with mortar rounds, while clashes broke out all around Ramadi, AFP reports.
"ISIS launched a surprise attack from four directions -- north, west, east and south of Ramadi," a police first lieutenant in the city told AFP by phone.
"The mortar fire has been continuous since midnight," police captain Qusay al-Dulaimi said.
Meanwhile the ISIS targeted key government buildings in the Centre of Ramadi, some 110 kilometers west of the capital Baghdad.
Adhal al-Fahdawi, a member of the Anbar provincial council, said Friday that IS had managed to capture part of an eastern district called Mudhiq but pro-government forces had stopped their advance and were encircling the terrorists there.
On the other side,new videos from the US Central Command released on Thursday showed recent US-led coalition airstrikes against ISIS targets in Iraq including Beiji and Kirkuk.
The US military said the airstrikes hit various targets including buildings, tunnels, and bunkers under the control of the ISIS group.
Mosques in Ramadi call for Defending against ISIS
ISIS terrorists attacked a government complex in the heart of Ramadi in an apparently coordinated effort to seize full control of the city.
Gunmen fired from rooftops at buildings in Ramadi housing the Anbar governorate offices and police headquarters, while security forces and tribal fighters tried to prevent the militants from advancing.
"Mosques are asking anyone who can carry weapons to confront the attackers," provincial council member Hathal Fahdawi told Reuters.
Militants also launched coordinated attacks to the east and west of Ramadi.