The Islamic State (Isis) has routed the government forces from Ramadi, the capital of Iraq's largest province Anbar, and hoisted the caliphate flags in the city.

In what has been considered as one of the biggest advances, the Islamists used car bombs to clear the path leading up to Ramadi. The bombings paved the way for the jihadists to storm government buildings including a military base of the provincial capital.

The governor's office and the main police headquarters have fallen under the control of IS militants after they used as many as six car bombs. Dozens of police officials have also been taken hostage by the insurgents.

A senior police officer told the AFP news agency IS "now occupies the government centre in Ramadi and has also raised its flag over the police HQ for Anbar". The IS has also formally claimed hoisting its flag atop the state buildings.

The latest militant push on Ramadi, located 100kms west of Baghdad, began on Thursday, 14 May with armoured bulldozers bursting through the gates of the government-held city.

Anbar, which makes up to about one-third of the Iraqi territory, is a strategically important province that has witnessed relentless violence between Iraqi security forces and the IS fighters.

"The situation in Ramadi is dire, but the city has not fallen and the battle against criminal Daesh is still ongoing," Sohaib al-Rawi, governor of Anbar province wrote on twitter.

The failure to secure the heavily-fortified complex in Ramadi is squarely blamed on the Baghdad administration and its lack of coordination among the government forces.

Baghdad officials fear other parts of Ramadi could also fall into the hands of IS if they continue to advance, marking the biggest victory for the militant group this year.

"If the government does not send any reinforcements and the coalition air force does not rescue us, I can confirm we will lose all Ramadi by midnight. 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," an army major, whose regiment is stationed near Anbar, told Al Arabiya.

The surprise attack was launched hours after a voice recording, purported to be of IS chief Abu Bakr al-Baghdadi, surfaced commanding the jihadists to step up their onslaught.