BEIRUT (Reuters) - Al Qaeda's Syrian affiliate, the Nusra Front, has evacuated its bases in populated areas of the Idlib region in northwest Syria after U.S.-led forces carried out air strikes on the group, its fighters said on Wednesday.

Another Syrian Islamist group, Ahrar al-Sham, has also ordered its followers to evacuate bases, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.

"Heavy weapons have been moved out of the bases. We do not want civilians to be harmed because of us," one Nusra fighter said in an online message posted on the Internet.

The Observatory also reported the Nusra withdrawal.

At least 50 fighters from the Nusra Front and eight civilians were killed in strikes by a U.S.-led coalition in Syria on Tuesday, the Observatory said.

The U.S. military said it had launched strikes on al Qaeda veterans it referred to as the "Khorasan group". Militants in Syria said the term was not widely used but may refer to fighters who came to Syria and joined the Nusra Front after fighting in Afghanistan.

The other Islamist group evacuating its bases, Ahrar al-Sham, is part of the Islamic Front alliance that has been in armed conflict with Islamic State militants, who have been the main targets of the U.S.-led air strikes.

The Observatory quoted a statement from Ahrar al-Sham as ordering followers to "evacuate bases, workshops and camps" except for what was needed to protect them.

The group also ordered followers to remove satellite and wireless communications devices from bases and refrain from using them unless "absolutely necessary", the Observatory said.

The statement called for heavy weapons to be hidden and warned civilians to stay away from the group's bases, the Observatory said.

This month an explosion killed the leader of Ahrar al-Sham and at least 28 of its commanders.

On Wednesday, the United States military said it had launched five more air strikes targeting Islamic State in Syria and Iraq in a latest round of attacks on the militant group which has seized swathes of territory in both Syria and Iraq.

(Reporting by Mariam Karouny and Alexander Dziadosz; Writing by Alexander Dziadosz, editing by Gareth Jones)