By Sylvia Westall
BEIRUT (Reuters) - The new head of the Islamist group Ahrar al-Sham, once one of the strongest militias in the Syrian civil war, has urged fellow insurgents to fight on after a blast on Tuesday wiped out its senior leadership.
In a video on YouTube, Ahrar al-Sham said Hashem al-Sheikh, also known as "Abu Jaber", had been named its new leader and Abu Saleh Tahan its new military chief. Another video showed a man identified as Abu Jaber exhorting his men to fight on.
The explosion in northwestern Syria killed at least 12 including Ahrar al-Sham's former leader Hassan Aboud, said the group which is part of the Islamic Front alliance fighting both the Syrian army and the now dominant Islamic State movement.
The Syrian Observatory for Human Rights, a group monitoring the conflict, said the blast killed 28 of Ahrar al-Sham's commanders, dealing a major blow to the organisation that is believed to have received funds from Gulf states.
"Jihadi men of our nation ... do not let the crisis shake you or the calamity divide you," Abu Jaber said on the video while eulogising the dead. "Rise, let us die for what they have died for," he said in a statement read from behind a desk.
Some 50 of the group's leaders had gathered at a house when the blast went off inside, according to the Observatory. There has been no claim of responsibility for the blast, which took place in Syria's Idlib province.
Some observers have described Tuesday's incident as a gas attack. Abu Baraa, a rebel figure from a group allied with Ahrar al-Sham, said a doctor who examined the bodies said there was little visible sign of external injuries.
The doctor saw bodies with frothing at the mouth and fluid coming from the eyes and noses, Abu Baraa said, adding the group had been meeting in a heavily fortified underground bunker.
"This was a highly sophisticated attack in a location that was very secure," he said. Photos posted on social media claiming to show the victims of the attack displayed bodies that did not appear to have significant external injuries.
Other reports suggested the victims had died from smoke inhalation. It was not possible to independently verify any of the reports or pictures or the cause of the deaths.
"CRIPPLING BLOW"
The Hague-based Organisation for the Prohibition of Chemical Weapons (OPCW) said in a statement on Wednesday it had found "compelling evidence" that chlorine gas was used "systematically and repeatedly" as a weapon in northern Syria this year.
President Bashar al-Assad agreed to hand over his chemical weapons stocks for destruction as part of a deal that averted threatened U.S. military strikes, but the continued attacks have led to accusations he had not fully declared his arsenal.
The loss of its senior leadership has come at a significant time for Ahrar al-Sham, said Charles Lister, visiting fellow at the Brookings Doha Centre.
"This potentially crippling blow comes during a period in which Ahrar al-Sham's senior officials have begun adopting more moderate stances in Syria, including considering joining larger moderate coalitions," he wrote in an analysis.
"The gutting of Ahrar al-Sham?s leadership will have major ripple effects in the opposition," researcher Aron Lund wrote on the Carnegie Endowment for International Peace's Syria website.
"Unless Ahrar al-Sham somehow manages to recover and sustain its relevance as a major Islamist faction, the Islamic Front may now be beyond repair," he wrote.
Some supporters of the rival group Islamic State celebrated Aboud's death on social media, saying that Syria had been "cleansed" of his presence.
In January another senior Ahrar al-Sham leader was killed in a suicide attack. Abu Khaled al-Soury had fought alongside al Qaeda founder Osama bin Laden and was close to its current chief Ayman al-Zawahri.
Islamic State, also known as ISIL, had denied involvement in the January attack after being blamed for it.
Ahrar al-Sham, which has advocated sharia law in Syria, was once considered among the strongest insurgent groups in the civil war but has since been overtaken by Islamic State.
The radical Islamic State group has seized wide swaths of Syria and Iraq and views other Islamist groups as its rivals.
U.S. Secretary of State John Kerry arrived in Baghdad on Wednesday as part of a tour of the Middle East aimed at building military, political and financial support to defeat Islamic State.
(Additional reporting by Suleiman al-Khalidi in Amman and Ahmed Tolba in Cairo; Editing by Tom Heneghan)
