US-led coalition troops have adapted their training for Iraqi forces to teach them how to breach Islamic State group defences in larger-scale combined assaults on jihadist strongholds.

IS, which overran large parts of Iraq in 2014, has saturated territory it controls with bombs, booby-traps and other obstacles that Iraqi forces must break through to drive the jihadists back.

Military operations in Ramadi, which was recaptured from IS at the end of last month, as well as other cities and towns showed the need for training in which soldiers combine different capabilities to breach jihadist defences.

The training "is based on the lessons that we learned watching the challenges in Ramadi... and Tikrit and Sinjar and Baiji," said Lieutenant General Sean MacFarland, the commander of the international operation against IS.

"It's not about defeating IEDs (improvised explosive devices); it's about breaching obstacles," MacFarland said following a training exercise at the Besmaya base near Baghdad.

The aim of live-fire exercises of this kind is to practice integrating "infantry, armour, engineers and indirect fire to overwhelm the enemy and combine the effects on an enemy force", he said.

Two brigades -- the 71st and 72nd -- have been through the new training, with the latter finishing the final exercise of more than two months of instruction on Wednesday.

During the exercise, 72nd Brigade soldiers combined mortar fire for smoke cover, engineers equipped with mine-clearing charges and bulldozers to open gaps in defences, and infantry in armoured vehicles to provide covering fire and then advance toward the objective.

A Mine-Clearing Line Charge fired from a trailer early in the exercise was the most dramatic weapon employed, sending a massive cloud of smoke and dust sweeping over the field as it opened a path for soldiers to advance.

But the soldiers also have a smaller clearing charge known as the APOBS -- Anti-Personnel Obstacle Breaching System -- as well as bulldozers to clear obstacles including the dirt berms that IS frequently erects.

- 'A conventional army' -

"The skills that they've been taught here now are all a product of stuff that we've bean learning from Ramadi," Brigadier James Learmont, the deputy commander of coalition ground forces in Iraq, said at Besmaya.

"You see what they've (the jihadists) got in Ramadi, these massive minefields which they put everywhere," he said. "Ramadi as a city is littered with improvised explosive devices, booby traps."

"We literally had to go back to the drawing board and say, 'OK, what do we need in order to actually defeat this?' And it is coming back to (this) breaching," he said.

For this mission, the engineers are key, but they cannot operate alone and unprotected, while infantry are also needed to capitalise on gaps in enemy defences.

"You can't just allow engineers to go forward, 'cause they're vulnerable," Learmont said.

"That's why you see the overwhelming application of fire... to give them the necessary support to keep the heads of (IS) down."

IS has moved on from the insurgent tactics of past years to become a force that holds territory and uses tactics like those of a regular army, the officers said.

"They're fighting very much like a conventional force," MacFarland said.

Learmont agreed, saying that IS "actually is a conventional army in its own right, and it's using weapons systems like a conventional army."

Coalition forces have been training Iraqi troops since 2014.

But the combined arms exercise at Besmaya -- where Spanish forces have been leading the training, with other countries also involved -- is a more advanced level of instruction.

"In the past... we concentrated on firing the rifle, basic skills. But now we've advanced past the basic skills to... what we would describe as collective training," Learmont said.

The Iraqi soldiers will soon be relying on that training in combat.

"We are preparing and training forces to fight (IS)," Learmont said. "They will literally finish here, and they'll go into the front line."