Coalition air strikes have hampered the Islamic State group's ability to launch suicide attacks across the key Iraqi city of Mosul, a British general said Wednesday.

Suicide missions, particularly those conducted in explosive-laden vehicles, have been a vital weapon for IS fighters trying to fend off Iraqi forces pushing to recapture the jihadists' last major IS stronghold in Iraq.

British Army Major General Rupert Jones, a deputy commander for the US-led coalition against IS in Iraq and Syria, said air strikes had focused on cutting routes used by the IS group to maneuver across the city.

So far, strikes have disabled four of the five bridges linking east and west Mosul, and peppered roads used by IS with craters, making them impassable.

"The intent of these operations is to reduce the effectiveness of the vehicle-borne improvised explosive devices," Jones told Pentagon reporters in a video call from Baghdad.

The bridges across the Tigris had not been totally destroyed, but partially damaged to make them unusable now but mendable in the future, he added.

"We are beginning to see a reduction in the amount of suicide attacks," Jones said, noting one reason was "damage to the bridges that is making it harder for (IS) to flow fighters and ammunition across the river."

The battle to retake Mosul began October 17, with coalition-backed Iraqi security forces pushing toward the city along multiple axes.

Progress was quick at first but slowed as troops entered the city proper, where they face grueling street-to-street battles.

The Iraqis say it could take up to six months to complete the operation, and Jones said plans were "broadly on schedule."

"My sense is that the enemy are beginning to struggle," he said.

Britain is one of 29 countries that has provided troops to help fight IS in Iraq and Syria.

"You do not liberate a heavily defended city the size of Mosul quickly, and patience is therefore needed."

General Joseph Votel, who heads the US military's Central Command, said separately at a Washington think tank event that the Mosul fight could take "a couple more months."

It is "generally on track," he said, while warning of a hard fight ahead.

The "Islamic State is fighting hard right now, but you have to look at the wear and tear that they are absorbing," he told the Foreign Policy Initiative.

"We are going to move at the pace of our partners and continue to keep the momentum going."