Iraqi forces have made significant advances around Ramadi, and an operation is looming to retake the city, captured by the Islamic State jihadist group in May, officers said Wednesday.

A similar advance was also reported on the Baiji front, with Iraqi forces retaking parts of the country's largest refinery and pushing north.

"Great people, the hour of victory against the Daesh (IS) criminal gangs has come," said the Joint Operations Command for Anbar province, of which Ramadi is the capital.

"Your heroic forces are advancing steadily from the northern side... they managed to reach Albu Farraj area," on the northern edge of the city centre, a statement said.

The head of the command, Major General Ismail Mahalawi, said "Iraqi forces have raised the Iraqi flag on Albu Farraj bridge", over the Euphrates River.

Since the start of October, Iraqi forces have been closing in on Ramadi, gaining ground west and north of the city in particular.

IS fighters took Ramadi, about 100 kilometres (60 miles) west of Baghdad, in mid-May after a three-day blitz of massive suicide car and truck bomb attacks that forced a disorderly retreat by pro-government troops.

After the most stinging setback suffered by Iraqi forces since they started a counter-offensive to regain territory lost in mid-2014, officials vowed to swiftly retake Ramadi.

Progress has been sluggish, however, with Iraqi forces and the US-led coalition supporting them in Anbar blaming a number of factors, including searing summer temperatures.

The coalition's spokesman in Baghdad, Colonel Steve Warren, conceded two weeks ago there had been an "operational pause" in efforts to retake Ramadi.

But on Tuesday he said Iraqi forces were now ready to launch an operation inside the city.

"We now believe that battlefield conditions are set for the ISF (Iraqi security forces) to push into the city," he said, estimating between 600 and 1,000 the number of IS fighters remaining in Ramadi.

- Fresh Baiji push -

According to the daily tallies provided by the US military, 58 air strikes have been carried out by coalition warplanes in the Ramadi area since the start of October.

Four were conducted on Tuesday, a statement said.

They "destroyed five ISIL buildings, five ISIL fighting positions, an ISIL tactical vehicle, two ISIL improvised explosive clusters used as minefields, and denied ISIL terrain at three separate locations," it said, using another acronym for IS.

The forces battling IS around Ramadi are mostly from Iraq's regular security forces, including the army, the police and the elite counter-terrorism services.

Thousands of Sunni tribal fighters from Ramadi and elsewhere in Anbar are also active on that front.

Other paramilitary outfits operating under the umbrella of the Hashed al-Shaabi, dominated by Shiite militia groups, have focused their efforts around Fallujah, which is still under IS control and lies about half way between Ramadi and Baghdad.

Army and Hashed fighters have also launched a broad offensive aimed at reviving efforts to recapture Baiji, a town about 200 kilometres north of Baghdad, and the nearby refinery.

The Joint Operations Command in Baghdad announced that several sectors of the refinery were recaptured on Wednesday from IS and that Iraqi forces were pushing north.

The Baiji refinery is the country's largest but is thought to have been damaged beyond repair by almost non-stop fighting since the first days of the offensive IS launched across Iraq in June 2014.

Baiji lies at a crossroads between several key fronts in the country, and a general in the Salaheddin province predicted the area would be fully reclaimed soon.

"In the coming hours, we will see a complete recapture of the entire Baiji region," he told AFP on condition of anonymity.

"It will secure operations to liberate Anbar to the west and Hawijah to the east, and pave the way for a push north towards Mosul," Iraq's second city, the officer said.

Sunni Arab tribal fighters from the Kurdish-controlled city of Kirkuk joined the battlefield Wednesday to help joint efforts to take back Hawijah, one of their leaders said.

Abu Ridha al-Najar said 200 of them, mostly from the Juburi and Obeidi tribes and under the command of the Hashed al-Shaabi, had deployed to areas southwest of Hawijah.