Suicide attackers from the Islamic State group killed at least 12 Iraqi forces Sunday in a brazen attack on police training at a military base, officials said.

A commando of fighters equipped with rifles and suicide vests snuck into Speicher base, near the city of Tikrit, in the middle of the night.

Their target was a large group of police forces from Nineveh, a northern province of which Mosul is the capital, who were undergoing training.

"Under the cover of fog, they broke into Speicher," said Mahmud al-Sorchi, spokesman for the paramilitary force being set up to take back IS-held Nineveh.

"Nineveh police managed to kill seven attackers but three were able to detonate their suicide vests," he said, adding that three officers were among the 12 policemen killed.

He also said 20 policemen were wounded in the attack.

Several other security sources in the region confirmed the attack, which was claimed by the Islamic State group.

The jihadist organisation said seven suicide attackers managed to enter the huge military base, which lies about 160 kilometres (100 miles) north of Baghdad.

In a statement posted online, IS said its commando reached a centre where 1,200 cadets were being trained, sparking clashes that lasted four hours.

Speicher is located in Salaheddin province, which was one of the regions conquered by IS when it swept across much of Iraq's Sunni Arab heartland in June 2014.

The sprawling military base itself was never fully controlled by the jihadists but at the beginning of their offensive they committed one of the conflict's worst atrocities there.

IS fighters assisted by local insurgents rounded up hundreds of cadets from Speicher, marched them to Tikrit and massacred them in several locations.

- Ramadi clearing ongoing -

Hundreds of bodies were discovered in shallow graves when the Iraqi forces retook Tikrit in April 2015 but other victims were shot and thrown into the Tigris and will likely never be found.

The highest estimates put the number of executed cadets at 1,700.

Security officials said Sunday raid was launched from the western side of the base, a desert area where IS remains able to operate despite the increased presence of Iraqi forces.

The group has launched a number of attacks since losing control of the city of Ramadi in the western province of Anbar a week ago.

After taking the strategic government complex in the centre of the city, elite counter-terrorism forces have been expanding their grip and sweeping each neighbourhood for holdout jihadists and trapped civilians.

A number of IS fighters remain in Ramadi but they are not in position to mount a fightback that would challenge the Iraqi forces' control of the city.

"Over the last 24 hours, the Iraqi forces have cleared several more hundred square metres of the city," said Colonel Steve Warren, spokesman of the US-led coalition whose trainers and aircraft helped Iraqi forces retake Ramadi.

"We don't think the enemy has enough combat power in downtown Ramadi to be able to recapture the city," he said.

"We haven't seen anything more than small teams of ISIL (IS) fighters (four to eight individuals) trying to conduct harassing attacks," Warren said.

The group has continued to carry out suicide attacks in various locations, including on Friday north of Ramadi and over the weekend near the other main cities of Anbar, Fallujah and Haditha.

Warren said at a recent briefing that months of daily strikes against the group, which the Iraqi government has lost half of the territory it took in 2014, are paying off.

"The enemy is getting extraordinarily desperate when it comes to manpower," he said.

The loss of Ramadi is the latest in a string of setbacks for IS, which analysts say is under increasing pressure.

The group has since released a flurry of statements that either claim responsibility for attacks or show scenes of daily life attempting to convey that IS's so-called "caliphate" is still strong.