WASHINGTON — A suicide bombing in Syria on Saturday killed Majed Hussein Al Sadeq, chief of staff of Ahrar al-Sham, one of most powerful organizations fighting the Islamic State group, Agence France-Presse reported.

Sadeq, along with three others members of the rebel group, were killed in an attack at their headquarters in Binnish, a town northeast of Idlib city, the Syrian Observatory for Human Rights, a monitoring group, told AFP.

It was not immediately clear who was responsible for the attack. The monitoring group told AFP an “unknown person” had parked a motorcycle near the headquarters, walked toward a group of fighters and detonated explosives.

The attack came the same week as the end of an almost two-month-long ceasefire in Syria. As IBT reported, opposition forces have requested that military operations resume against the regime of Syrian President Bashar Assad and that the United Nations delay peace talks.

Meanwhile, Iran is looking to increase its presence in Syria. “They [the Iranians] saw it as an opportunity to move closer to the regime,” one U.S. official told the Financial Times.

In Syria, opposition groups have been launching new offensive strategies. For instance, Ahrar al-Sham and other opposition groups in the Free Syrian Army have worked to seize regime-held towns in Latakia, including Nashbba, Rasha and al-Malik.

Formed in 2011, Ahrar al-Sham is reportedly funded by Turkey and countries in the Persian Gulf region. The U.S. has kept its distance from the group given its ties to al Qaeda.

Sadeq reportedly held several positions in the group before he became chief of staff.