The top US commander in Iraq on Tuesday acknowledged the likelihood that the US-led coalition played a role in blasts in Mosul that killed many civilians this month, but said an investigation was under way and ISIL may also be to blame.

"My initial assessment is that we probably had a role in these casualties ... What I don't know is were they [the civilians] gathered there by the enemy? We still have some assessments to do," Lieutenant-General Steve Townsend told a Pentagon news briefing, speaking from Iraq.

"I would say this, that it sure looks like they were."

Conflicting accounts have emerged since the March 17 explosion in al-Jadida district in west Mosul, where Iraqi forces, backed by US-led coalition air strikes, are fighting to clear Islamic State of Iraq and the Levant (ISIL) fighters from Iraq's second city.

Investigators are in Mosul to determine whether a US-led coalition strike or ISIL-rigged explosives caused a blast that destroyed buildings and may have killed more than 200 people.

"My initial impression is the enemy had a hand in this. And there's also a fair chance that our strike had some role in it," Townsend said.

"I think it's probably going to play out to be some sort of combination. But you know what, I can't really say for sure and we just have to let the investigation play out."

More than 300 civilians have been killed in west Mosul since Iraqi forces and the US-led coalition began an offensive last month to push ISIL out of its last stronghold in Iraq, the UN said on Tuesday, adding the toll could exceed 400 if new killings are verified.

"This is an enemy that ruthlessly exploits civilians to serve its own ends, and clearly has not even the faintest qualm about deliberately placing them in danger," UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein said in a statement.

"[ISIL's] strategy of using children, men and women to shield themselves from attack is cowardly and disgraceful. It breaches the most basic standards of human dignity and morality," he said.

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Hundreds of thousands more civilians are still trapped inside west Mosul after Iraqi forces and the US-coalition recaptured the city's east from ISIL in January.

West Mosul is both smaller and more densely populated than the city's east, meaning this stage of the battle poses a greater danger to civilians than those that came before.

Amnesty International's Donatella Rovera said field research in east Mosul showed "an alarming pattern of US-led coalition air strikes, which have destroyed whole houses with entire families inside".

"The high civilian toll suggests that coalition forces ... have failed to take adequate precautions to prevent civilian deaths, in flagrant violation of international humanitarian law," she said.

In the east, the Iraqi forces adopted a strategy of encouraging civilians to stay at home, dropping leaflets into the city with safety instructions for residents.

"The fact that Iraqi authorities repeatedly advised civilians to remain at home instead of fleeing the area indicates that coalition forces should have known that these strikes were likely to result in a significant numbers of civilian casualties," Rovera told Al Jazeera.

Witnesses told Amnesty that people were killed in their own homes after heeding government advice not to flee the city.

The UN said it also received reports of another 95 people killed in four western Mosul neighbourhoods between March 23 and 26.

The rights office said it was not in a position to provide a breakdown of the deaths caused by ISIL violence and air strikes by the international anti-ISIL coalition.