US special forces soldiers have killed a senior leader of the Islamic State of Iraq and the Levant (ISIL) group during a raid carried out in eastern Syria, the US secretary of defence said.

Pentagon chief Ash Carter said in a statement on Saturday that the raid in al-Amr had killed Abu Sayyaf when he "engaged US forces".

Al Jazeera's Omar al-Saleh, reporting from the Bekaa valley in neighbouring Lebanon, said two US helicopters had taken part in the operation at an oil field east of the city of Deir Az Zor.

"Activists from the area say at least six people from ISIL were killed, including at least two of Arab nationalities; a Saudi and an Iraqi," Saleh said.

"We believe the operation may have taken place overnight or early Saturday."

The US put the number of ISIL deaths at about a dozen, and said no US soldiers were killed during the incident.

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Abu Sayyaf was involved in planning ISIL's military operations and had helped direct its oil, gas, and financial operations.

His wife was captured during the operation and is currently being detained by the US military in Iraq.

Another White House official, Bernadette Meehan, said a Yazidi woman kept as a slave by Sayyaf and his wife was rescued during the raid, which was authorised by US President Barack Obama.

"This operation was conducted with full consent of Iraqi authorities and, like our existing air strikes against ISIL in Syria, consistent with domestic and international law," Meehan said.

The White House said no advance notice was given to the Syrian government about the operation, the Reuters news agency reported.

"We have warned the Assad regime not to interfere with our ongoing efforts against ISIL inside of Syria," Meehan said.

Bill Fallon, the former head of the US Central Command, told Al Jazeera that the operation showed the US had made advances in collecting intelligence on ISIL.

"It puts ISIL on notice that they'll have to be wary of the fact allied forces could come and get them at any time," Fallon said.

"The major objective of these kinds of things is to keep ISIL off balance, to let them know there's no sanctuary no matter where they are, that will be very helpful to the effort on the ground."