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The Pentagon ordered U.S. military installations to heighten their security levels over concerns related to the Islamic State militant group. Reuters

The United States has carried out an airstrike in Syria that killed a prominent leader of al Qaeda offshoot Nusra Front, Abu Firas al-Suri, U.S. officials told Reuters on Monday.

Islamic rebel sources said Abu Firas, who was a former Syrian army officer discharged in the late 1970s because of his Islamic leanings, was a founding member of the militant group and had fought in Afghanistan in the 1980s.

He was a senior member of the Nusra Front's policymaking Shura Council.

A Pentagon spokesman said a U.S. airstrike Sunday hit a meeting of high-level al Qaeda officials in northwest Syria at which Abu Firas was present. The spokesman, Peter Cook, said the United States was still confirming whether Abu Firas had been killed.

The United States has targeted the Nusra Front in the past, although the bulk of the U.S. military's firepower in Syria has been directed at the Islamic State group, aka ISIS, which occupies parts of Syria and Iraq.

Abu Firas was a fervent opponent of ISIS' style and was ideologically at odds with the militant group.

The Syrian Observatory for Human Rights, which tracks violence in the country, had said he was killed in a suspected Syrian or Russian air raid on a village northwest of the city of Idlib in northwestern Syria on Sunday.

But rebel sources said the attack appeared to have the hallmarks of a U.S. drone strike.

The U.S. officials, who spoke on condition of anonymity, declined to offer details on the strike itself, including whether it was carried out by manned or unmanned aircraft.

They also did not say whether the Nusra Front leader was specifically targeted.

From Madaya, near Damascus, Abu Firas worked with al Qaeda leader Osama bin Laden in galvanizing support among Pakistani supporters of the fundamentalist Taliban movement in Afghanistan several decades ago, the rebel sources said.

"May God accept him as a martyr; he was a commanding figure. This was engineered by the Crusader axis," one of the sources said.