Two top leaders of the terrorist al-Qaeda organisation were killed in separate military operations in two countries this month, the US Department of Defense announced.

The first killed was Muhsin al Fadhli, a high-ranking al-Qaeda official who was killed during a US airstrike two weeks ago while traveling in a vehicle in northwestern Syria, the Pentagon said.

On Friday, the Pentagon announced that a second US airstrike, this time in Afghanistan, killed another high-ranking al-Qaeda operational commander identified as Abu Khalil al-Sudani.

The Pentagon said he was killed in a July 11 strike in the Bermal district of Afghanistan's Paktika province.

It said al-Sudani was the head of al-Qaeda "suicide and explosives operations" and was linked to external attack plotting against the United States.

Al-Fadhli, the earlier target of a US airstrike, was a Kuwaiti-born, high-level terrorist, who had a $7-million reward on his head, said Pentagon spokesperson Capt. Jeff Davis in a written statement.

He was the leader of the Khorasan Group, composed of senior al-Qaeda members who have transferred into Syria, Davis said.

"His death will degrade and disrupt ongoing external operations of al-Qaeda against the United States and our allies and partners," the Pentagon spokesperson said.

It was not made clear, however, if he was killed by a drone or a piloted aircraft, said The Guardian.

Davis said the airstrike was launched on July 8 while the al-Qaeda leader was travelling in a vehicle near Sarmada in Syria.

The Khorasan Group led by al-Fadhli is believed to be the brains plotting attacks in Western countries while Islamic extremist groups like ISIS and the al Qaeda-linked Jabhat al-Nusra are blamed for most of the chaos inside Syria's frontiers.

One of the Khorasan Group's facilities was hit by an air assault last September. Coalition aircraft have been attacking Khorasan's training camps and other facilities inside Syria since last year.

Al-Fadhli fought alongside al-Qaeda and Taliban militants in Afghanistan, Davis said. He was also among the "few trusted al-Qaeda leaders who received advanced notification of the Sept. 11, 2001 attacks in the US," he added.

Al-Fadhi had been linked in terrorist attacks since October 2002, one against US Marines on Faylaka Island in Kuwait and another against the French ship MV Limburg.