The world's chemical watchdog on Wednesday confirmed the "systematic" use of chlorine as a weapon in Syria, according to a report by its team investigating alleged attacks there.

The fact-finding mission established “compelling confirmation” that a toxic chemical was used “systematically and repeatedly” as a weapon in villages in northern Syria earlier this year, the Organization for the Prohibition of Chemical Weapons (OPCW) said in a statement.

A blast has wiped out the leadership of a pragmatic Islamist rebel brigade in Syria, dealing a new blow to the country's armed opposition that could splinter its most important coalition.

Late Tuesday 47 members of Ahrar al-Sham's leadership were killed when a blast hit a meeting of its top religious and military chiefs in the northeastern province of Idlib, including its chief Hassan Abboud, known by the name Abu Abdullah al-Hamawi.

In finally tackling the jihadist threat in Syria and Iraq, Washington and Gulf Arab states are confronting a "monster" that has come back to bite them, partly through their own failings, analysts say.

After a stop Wednesday in Baghdad, U.S. Secretary of State John Kerry will hold talks Thursday with Arab states and Turkey in Saudi Arabia to broaden an international alliance set to act against the Islamic State movement (IS).

Austrian police prevented two schoolgirls aged 14 and 15 from travelling to Syria, officials said Wednesday with press reports saying they wanted to join Islamist guerrillas.

Foreign Minister Jebran Bassil met on Wednesday with his visiting Danish counterpart Martin Lidegaard who is in the region to strengthen cooperation against Islamic State extremists.

Bassil told reporters that he stressed during their meeting “the importance of building an international united front to face the spread of terrorist groups in the region.”

Staffan de Mistura, on his first Damascus visit as the new U.N. envoy on the Syrian conflict, is to hold talks with President Bashar Assad on Thursday, a newspaper close to the regime said.

Al-Watan said de Mistura, who arrived on Tuesday, was sounding out officials in Damascus on the prospects for a political settlement to the more than three-year-old conflict.

The relatives of soldiers and policemen held captive by Islamists since August staged a sit-in on Wednesday in Beirut's Martyrs' Square, demanding that the government exert all possible efforts to ensure the release of their loved ones.

Mustaqbal bloc MP Khaled Zahraman declared from the square: “Political exploitation of this file will not help their cause.”

Syria's Islamist Ahrar al-Sham rebel brigade named new chiefs on Wednesday after a devastating blast killed nearly 50 members of its leadership in a northwestern province largely under rebel control.

The blast in Idlib province on Tuesday evening killed the key rebel group's leader Hassan Abboud, known by the name Abu Abdullah al-Hamawi, along with 46 other top members.

Australian police Wednesday raided an Islamic centre and seized two men for alleged involvement in recruiting and sending jihadist fighters to Syria.

Israel on Wednesday hailed U.S. efforts to form an alliance to fight jihadists in Syria and Iraq but raised concerns of a rapprochement between Washington and Israel's arch-foe Iran.