Ahrar al-Sham, a Syrian rebel group, has announced its new leadership after it lost its top brass during a bomb attack on a high-level meeting in Idlib province.

Hassan Abboud, the head of Ahrar al-Sham, was among up to 45 people killed in an underground bunker near an ammunition dump outside Ram Hamdan on Tuesday.

The new head is Hashim al-Sheikh, also known as Abu Jaber, while his deputy is Abu Saleh Tahan, the group announced on Wednesday.

It pledged to keep fighting against Syrian government forces and the Islamic State armed group, Reuters news agency said.

Along with Abboud, about 50 of the group's leaders had been gathered at a house when the blast went off, according to the Syrian Observatory for Human Rights, a group that monitors the conflict.

There has been no claim of responsibility for the blast.

The Idlib meeting brought together Ahrar and a number of other opposition groups fighting as the Islamic Front alliance, including Abdallah Azzam and the Iman brigades, to discuss a strategy to confront the Islamic State.

Activists said others killed in the attack were Abu Yazan al-Shami, a member of the Ahrar's shura council; military field commanders Abu Talha al-Askari and Abu Yousuf Binnish; and Abu al-Zubeir, the head of the Iman brigade.

'Infiltration possible'

Abu al-Mustafa al-Ambsi, a member of the political bureau of Ahrar, told Al Jazeera the group was investigating the attack.

"There is a possibility that the meeting was infiltrated and an explosion happened first in the bunker," he said. "Maybe someone planted a device inside because the bunker is at a secret location.”

He said that killing of such an elite group "will only make us more resilient to fight and continue the fight until we liberate our homeland".

It is not known who staged the attack but Islamic State sympathisers hailed the death of Abboud on social media.

Another of Ahrar's leaders, Abu Khaled al-Souri, a close associate with al-Qaeda leaders Osama bin Laden and Ayman al-Zawahiri, was killed by the Islamic State earlier this year.

That assassination caused the schism that pitted the Islamic State against other rebel factions in Syria.

Ahrar has about 20,000 fighters and is the main force in the Islamic Front alliance, which was formed earlier this year to oppose the Islamic State group besides battling the forces of President Bashar al-Assad.

Ahrar advocates for a state run on Islamic principles, which protects the rights of women and religious and ethnic minorities, and disagrees with the approach of the Islamic State group.

In an interview with Al Jazeera in December 2013, Abboud said he would fight for his rights and dismissed UN-brokered talks in Geneva between the Syrian government and the exiled opposition umbrella Syrian National Coalition.

However, the rise of the Islamic State after the Geneva talks gave the civil war a new dimension, with Ahrar fighting not only the government but other rebel groups.