The White House has confirmed Islamic State second-in-command Hajji Mutazz has been killed in a US military air strike.

“Fadhil Ahmad al-Hayali, also known as Hajji Mutazz … was killed in a US military air strike on August 18 while travelling in a vehicle near Mosul, Iraq, along with an ISIL media operative known as Abu Abdullah,” White House spokesman Ned Price said.

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“Al-Hayali’s death will adversely impact ISIL’s operations given that his influence spanned ISIL’s finance, media, operations, and logistics,” Mr Price said, using another name for the group.

The White House said the dead leader was a “primary coordinator” for moving weapons, explosives, vehicles, and people between Iraq and Syria.

He was in charge of operations in Iraq and helped plan the group’s offensive in Mosul in June of last year.

The news comes as fragments from mortars fired by IS militants at Kurdish fighters in northern Iraq earlier this month tested positive in a US military field test for the chemical weapons agent sulfur mustard.

The United States and its allies stage daily air strikes on Islamic State targets in the group’s self-declared caliphate in Iraq and Syria.

A drone strike last month killed a senior Islamic State leader in its Syrian stronghold of Raqqa.

One counter-terrorism specialist cautioned that the impact of the killing on Islamic State could be short-lived.

“My experience in looking at the Islamic State suggests they have demonstrated an ability to move people up into positions” when high-ranking operatives are killed, said Seth Jones, a former Pentagon official.

Jones said how much territory Islamic State controls was more important in determining the group’s power.

“The key issue is territorial control,” he said.

Meanwhile, a spokesman for the United States-led coalition fighting Islamic State says he would welcome Australia’s involvement in Syria.

Australia is already part of the coalition — also US-led — conducting air strikes against Islamic State militants in Iraq.The Australian Government has been considering a request from the Pentagon to fly across the border into Syria.

Chief of staff of Operation Inherent Resolve Brigadier General Kevin Killea said extra help would be welcomed.

“Their contributions to date have been enormous,” he said.

Prime Minister Tony Abbott confirmed he had been in talks with Coalition partners about Australia extending airstrikes against the militant group into Syria.

Obviously there have been some approaches made at various levels,” he said.

“We shouldn’t get ahead of ourselves on this because no formal decision has been made and no formal request has been taken.”