JAKARTA/MANILA (Reuters) - Indonesia is investigating reports from Islamic State supporters that the most senior Southeast Asian commander of the militant group was killed by U.S. air strikes in eastern Syria last week, counter-terrorism officials said.

Online messages from Islamic State propagandists viewed by Reuters say Bahrumsyah, an Indonesian national, died after U.S. air strikes hit Hajin, north of the Syrian city of Abu Kamal, last Tuesday.

A spokesman for Indonesia’s foreign ministry, Arrmanatha Nasir, said the embassy in Syria had made enquiries but had yet to confirm Bahrumsyah’s death.

Two senior Indonesian counter-terrorism officials, speaking on condition of anonymity, said they were taking the online reports seriously.

“We are in the process of investigating,” said one senior official with Indonesia’s counter-terrorism agency.

If the reports were true, it would become a “motivation to carry out reprisal attacks” in Indonesia, the senior official said.

A Pentagon spokesman, Eric Pahon, said U.S. aircraft were bombing the “general area” in eastern Syria on the day Bahrumsyah is believed to have died but was unable to confirm his death.

As well as leading Katibah Nusantara, an armed unit comprising more than 100 Southeast Asians, Bahrumsyah also organized funding for the Islamist rebels who captured part of the southern Philippines city of Marawi in a bloody siege last year, analysts and officials say.

A message purportedly from the Islamic State figure Abu Nuh reviewed by Reuters said Bahrumsyah had been attending a meeting of leaders when he was killed. An Islamic State headquarters and a vehicle-borne improvised explosive device factory were destroyed in the attack, the message said.

Another post eulogized the Indonesian, receiving sympathetic comments and crying emojis.

There were reports last year of Bahrumsyah’s death, but analyst Sidney Jones from the Jakarta-based Institute for Policy Analysis of Conflict said the latest had a “much higher degree of credibility”.

“As far as we know, he was the highest ranking Indonesian to fight with ISIS. The fact that he commanded a fighting unit that was recognized by ISIS underscores his importance,” said Jones, using an alternative acronym for Islamic State.

His death, if confirmed, would be a blow to pro-Islamic State forces in Southeast Asia, where fears of hardened fighters returning from Syria as the militants’ self-declared caliphate crumbles has authorities on alert.

More than 600 Indonesians, including at least 166 women and children, traveled to Syria to join Islamic State, according to data from Indonesia’s counter-terrorism agency reviewed by Reuters.

A further 482 Indonesians were deported by foreign governments trying to join IS.

“I don’t expect a flood of people to come back (to Indonesia), although there will be some people trying,” Jones said.