Ayad al-Jumaili, believed to be ISIL's second-in-command after Abu Bakr al-Baghdadi, has been killed in Iraqi air strikes near the Syrian border, Iraqi state TV reported.

Jumaili and two other commanders were reportedly killed in the al-Qaim region in the western Anbar province.

Brigadier general Yahya Rasool of Iraq's Joint Operation Command, citing military intelligence, said the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group's director of administrative affairs, Salem Mudafar al-Ajmi, was also killed in the raid.

The news comes as the Iraqi government continued its push to liberate more areas from ISIL control in Mosul.

More than 300 civilians have been killed in western 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, adding that 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.

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.