AMMAN (Reuters) - A Western-backed rebel leader involved in the fight against Islamic State in southern Syria was killed by a suicide bomber suspected to be from the ultra-hardline group, rebel sources said on Thursday.

Saleem Bakour, a defector from the Syrian army where he was a colonel, was killed when a militant managed to get into his heavily guarded base in the desert near the Jordanian-Iraqi border and blew himself up. He was buried in Amman on Thursday.

Bakour, a founder of the Free Syria Army’s Southern Front alliance of rebel groups, took a lead in recent fighting to drive out Islamic State militants when they pushed south after being driven out of the central city of Palmyra in March.

Unlike parts of rebel-held northern Syria, Syria’s south has long been one of the last major footholds of rebel groups that are not dominated by hardline jihadists.

A rebel source said Bakour played a crucial role in April in pushing out Islamic State militants when they launched an attack on northeast Damascus around Dumier airport and abducted scores of workers.

“The martyr was one of the toughest leaders who fought Daesh (Islamic State). We are committed to fighting them to the end,” said Issam el-Rayyes, the spokesman for the FSA’s Southern Front told Reuters.

Bakour, a member of the mainstream opposition body the Saudi-based High Negotiations Committee (HNC), was among the rebel leaders who coordinate operations from a joint command center in Jordan that provides support to Syrian rebels provided by foreign states, including Gulf Arab governments.

This year, these moderate rebel groups have fought their biggest offensives in the south against hardline groups such as Liwa Shuhada al Yarmouk, suspected of ties with Islamic State, in ongoing battles that have killed dozens.

Fighting against the Syrian army has generally subsided as moderate rebels increasingly give priority to fighting Islamic State affiliated groups in the south.