Iraqi forces defused booby traps and hunted down holdout jihadists in the strategic Baiji area Friday as part of their biggest advance against the Islamic State group in months.

Baiji lies at a crossroads between several frontlines, and control of the area is seen as the key to progress in other regions, including Anbar province where forces were also closing in on IS strongholds.

The army, police and counter-terrorism services, as well as thousands of fighters from the Popular Mobilisation (Hashed al-Shaabi), continued to gain significant ground in and around Baiji, officers said.

"Iraqi forces are moving deep into Baiji; they have retaken the industrial area and several other neighbourhoods," an army colonel told AFP.

"We control about 60 percent of the city; there are not so many Daesh fighters left and they are trapped," he said, using an Arabic acronym for IS.

After retaking most of the refinery to the north of the city, security forces were sweeping the sprawling complex for bombs and die-hard jihadists.

"Inside the refinery, our forces are defusing booby traps and looking for the last Daesh terrorists we believe are still holed up in some buildings," he said.

The refinery, which once produced 300,000 barrels per day of refined products meeting half of Iraq's needs, is said to have been damaged beyond repair and to no longer be of huge strategic interest.

At least six anti-IS fighters were killed at the refinery Thursday, several officers said.

The bodies of at least 15 IS fighters were also found there and large numbers of wounded jihadists are reported to have been evacuated to the nearby IS strongholds of Hawijah and Sharqat.

The same officer also said loyalist forces had completely surrounded Sinniya, a town west of Baiji on the road leading to Anbar.

"We are firing large numbers of rockets and missiles, while Iraqi warplanes are also striking. This will prepare the ground for an operation to cleanse Sinniya," he said.

- Anbar operations -

The operation launched this week to secure Baiji, which has seen almost uninterrupted fighting since IS swept across Iraq's Sunni Arab heartland in June 2014, appears to be spearheaded by the Hashed.

Hadi al-Ameri, the most visible commander of the Hashed and a leading member of the Tehran-backed Shiite militia Badr, has been omnipresent on the Baiji frontlines.

Qassem Soleimani, commander of the foreign wing of Iran's elite Revolutionary Guards, was also reported in Iraqi media to have played a key role.

The US-led coalition, which is more active on the Anbar front, said Friday it had carried out two strikes in the Baiji area the previous day.

It also said it had destroyed parts of another refinery, in Qayyarah, between Baiji and main northern city Mosul, that "was used by Daesh to produce oil for the black market to fund their terrorist activities".

Around Ramadi, which IS seized in mid-May, Iraqi forces pressed an offensive to retake it, with air support from the coalition.

According to daily tallies provided by the US military, coalition warplanes have carried out at least 69 strikes against IS in the Ramadi area this month alone.

Major ground advances in recent days have allowed pro-government forces to almost completely encircle Ramadi, where the coalition estimates the number of remaining IS fighters to be between 600 and 1,000.

"We're encouraged by this progress, but much fighting lies ahead. Important for Iraq forces to keep moving forward," US military spokesman Patrick Ryder said on social media.

Iraqi forces were also tightening the noose around IS in the town of Baghdadi, further west up the Euphrates River in Anbar province.

Army Major General Ali Dabbun said his forces, with backing from local Sunni tribal fighters opposed to IS, had made good progress in Baghdadi.

"Our forces defused 62 IEDs (improvised explosive devices), blew up three rigged buildings, causing no victims," he said, claiming that many IS fighters had been killed in the latest fighting.

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