BEIRUT (AFP) - Fighters from the Islamic State in Iraq and Syria (ISIS) group executed at least 17 people, including civilians, after the jihadists seized Syria's ancient city of Palmyra on Thursday, a monitor said.

"ISIS executed 17 people, including civilians and loyalist fighters. At least four of them were beheaded," Rami Abdel Rahman, director of the Syrian Observatory for Human Rights monitoring group, told AFP.

He said the civilians were working for a local administrative council, while the fighters included at least one regime soldier and members of the National Defence Forces, a pro-government militia.

"They were accused of working with the regime," Abdel Rahman said.

A Syrian activist told AFP via Facebook that ISIS ordered residents to stay indoors.

"ISIS is preventing residents from leaving their homes and is combing civilian houses" to find regime loyalists, said the activist who calls himself Mohammad Hassan al-Homsi.

He said he had spoken by phone to people in Palmyra who told him they wanted to flee the city but were afraid of doing so after hearing about the executions, especially the reported beheadings.

According to the Observatory, at least 460 people were killed in the battle for Palmyra that began on May 13, including 49 who were executed by the militant group, nine of them children.