By Sylvia Westall

BEIRUT (Reuters) - Iraqi pilots who have joined Islamic State in Syria are training members of the group to fly in three captured fighter jets, a group monitoring the war said on Friday, saying it was the first time the militant group had taken to the air.

The group, which has seized swathes of land in Syria and Iraq, has been flying the planes over the captured al-Jarrah military airport east of Aleppo, said Rami Abdulrahman, who runs the Britain-based Syrian Observatory for Human Rights.

Reuters was not immediately able to verify the report and U.S. Central Command said it was not aware of Islamic State flying jets in Syria.

U.S-led forces are bombing Islamic State bases in Syria and Iraq. The group has regularly used weaponry captured from the Syrian and Iraqi armies and has overrun several military bases but, if the report is confirmed, this would be the first time it has been able to pilot warplanes.

"They have trainers, Iraqi officers who were pilots before for (former Iraqi president) Saddam Hussein," Abdulrahman said.

"People saw the flights, they went up many times from the airport and they are flying in the skies outside the airport and coming back," he said, citing witnesses in northern Aleppo province near the base, which is 70 km (45 miles) south of the Turkish border.

Witnesses reported the flights were at a low altitude and only lasted five to 10 minutes before landing, the Observatory said. It was not possible to reach the Syrian government for comment and state media did not mention the report.

It was not clear whether the jets were equipped with weaponry or whether the pilots could fly longer distances in the planes, which witnesses said appeared to be MiG 21 or MiG 23 models captured from the Syrian military.

U.S. MILITARY KEEPING "CLOSE EYE"

"We're not aware of ISIL conducting any flight operations in Syria or elsewhere," U.S. Central Command spokesman Colonel Patrick Ryder said, using a former name for Islamic State.

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"We continue to keep a close eye on (Islamic State) activity in Syria and Iraq and will continue to conduct strikes against their equipment, facilities, fighters and centers of gravity, wherever they may be."

General Lloyd Austin, head of the U.S. military’s Central Command, said he could not confirm that Iraqi pilots had joined Islamic State.

"We don’t have any operational reporting of ISIL flying jets in support of ISIL activity on the ground and so I cannot confirm that. And to the degree that pilots may have defected and joined the ranks of ISIL, I don’t have any information on that either," he told a Pentagon news briefing.

Pro-Islamic State Twitter accounts have previously posted pictures of captured jets in other parts of Syria but the aircraft appeared unusable, according to political analysts and diplomats.

The countryside east of Aleppo city is one of the main bases of Islamic State in Syria. The al Qaeda offshoot controls up to a third of the territory of Syria, whose civil war pitting various rebel groups against the regime of President Bashar al-Assad has raged for more than three years.

(Additional reporting by Oliver Holmes in Beirut and Phil Stewart and David Alexander in Washington; Editing by Gareth Jones)