US kills Islamic State's second in command Haji Imam   Abd al-Rahman Mustafa al-Qaduli
Iraqi soldiers fire a rocket toward Islamic State militants on the outskirts of the Makhmour south of Mosul, Iraq, March 25, 2016 Reuters

Abd al-Rahman Mustafa al-Qaduli, also known as Haji Imam, believed to be the second highest ranking commander of the Islamic State in Syria, has been killed in a US raid.

US defence secretary Ash Carter said on Friday al-Qaduli, an Iraqi, was serving as finance minister for the jihadist group and also responsible "for some external affairs and plots."

Along with al-Qaduli, the US forces also killed Abu Sarah, who Carter said was charged with paying fighters in northern Iraq.

The mandate of the US operation on al-Qaduli was to 'capture or kill' and the mission become a kill operation when he opened fire on the US helicopter.

However, Carter did not confirm this, nor did he confirm if Haji Imam was killed in a drone operation.

"I'm not going to say where and how it was done. But the only thing I will say it is consistent with our strategy there which is to put pressure on Isil every single way we can, from the leadership right down to supporting local forces on the ground."

US kills Islamic State's second in command Haji Imam   Abd al-Rahman Mustafa al-Qaduli
Abd al-Rahman Mustafa al-Qaduli is seen in an undated picture from the U.S. Department of State. Al-Qaduli, the second-in-command of the Islamic State, was killed in Syria.

The 57-year-old carried a bounty of seven million offered by the US for his head, the second highest after one million offered for information leading to the capture of Isis chief Abu Bakr al-Baghdadi.

"It's a big blow to IS. He has been an important figure going back to a decade ago during Abu Musaib Zarqawi's era in creating what became ISIS," he said, using another acronym for the group," Bruce Riedel, a former CIA analyst and Middle East expert at the Brookings Institution, told Reuters.

Carter said the killing of the senior Islamic state leader was in line with the US plan of "systematically eliminating" the terror group's cabinet.

Earlier this month, the US had killed Omar al-Shishani, known as "Omar the Chechen," who was defence minister of the jihadist group hich controls vast swathes of land in Syria and Iraq.

"The momentum of this campaign is now clearly on our side."

"These leaders have been around for a long time. They are senior, they're experienced, and so eliminating them is an important objective and it achieves an important result. But they will be replaced and we'll continue to go after their leadership and other aspects of their capabilities," Carter said.