Suicide attackers stormed a gas plant north of Baghdad Sunday, killing at least seven people and setting fire to tanks in the latest such assault claimed by the Islamic State group.

Brett McGurk, the top US envoy to the coalition battling IS, said meanwhile that the increase in such attacks showed the jihadists were under pressure in the face of the offensive against it.

The attack on the Taji plant, about 20 kilometres (12 miles) north of the capital, was launched at dawn.

Eight attackers broke into the plant and set off a car bomb at one of its entrances, said Iraqi interior ministry spokesman Saad Maan.

Some attackers detonated suicide belts and others were killed by bullets, according to Maan, who said explosions set three gas storage tanks alight.

Plumes of black smoke billowed into the sky but the fire was quickly brought under control.

The attack killed at least seven people and wounded at least 22, security and medical officials said.

In an online statement, IS said the attack was carried out by four suicide bombers, for whom it provided noms de guerre.

"They killed the guards at the gate before raiding the headquarters and killing all inside," the statement said.

When additional Iraqi forces massed at the gate for an attack, "our brothers detonated a car bomb in the middle of their gathering," and the militants clashed with security forces and detonated explosive belts among them.

The jihadist group estimated the number of dead and wounded at 45.

IS has been steadily losing ground to the Iraqi security forces in recent months.

According to the government, IS controls only 14 percent of Iraqi territory, down from the 40 percent it held in 2014.

But the group has intensified its attacks behind the front lines, detonating car bombs in civilian areas and infiltrating sensitive sites with suicide commandos.

"Daesh (IS) is turning to targeting civilian facilities in cities after losing the battle on the front," said Colonel Mohamed al-Bidhani of the government's "war media cell".

On Saturday, a group of IS fighters sneaked into Amriyat al-Fallujah, a government-held town west of Baghdad, in a similar suicide raid that killed five people.

The group also claimed responsibility for a spate of bombings in Baghdad on Wednesday that killed close to 100 people, the bloodiest day in the capital this year.

- 'Constant, synchronised pressure' -

Speaking at a conference in the Jordanian capital, US envoy McGurk said IS had "returned to suicide bombing" because the area under its control was shrinking and it was on the defensive.

"We are now making progress against Daesh," McGurk said, using an Arabic acronym for IS.

"The campaign to isolate and squeeze and constrict Daesh in Mosul has already began," he said of Iraq's second city and IS's main hub in the country.

"We are doing precision air strikes in Mosul almost every day, we have a lot of information from the people who are inside Mosul about what Daesh is doing inside the city."

The Iraqi army said in late March that its troops and allied militia had launched what was expected to be a long and difficult offensive to retake Mosul.

McGurk said IS was now under "constant, synchronised pressure".

"Their territory is shrinking and they are now doing these suicide attacks against civilian populations," he said.

Iraqi forces collapsed in the face of the 2014 IS advance and the jihadist group ultimately overran around a third of the country.

The US-led coalition of Western and Arab nations launched air strikes against IS in Iraq in August of 2014 and has killed thousands of the jihadists.

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