DAMASCUS, Syria—Syrian government forces used warplanes, helicopters and artillery on Sunday to pound districts of the capital held by the Islamic State group, in a bid to enforce an evacuation deal reached with the militants earlier in the week.

The militants agreed to give up their last pocket in southern Damascus on Friday but have yet to begin surrendering to government forces and relocating to IS-held areas elsewhere in the country.

State-run al-Ikhbariya TV showed thick gray smoke billowing from the IS-held Hajar al-Aswad neighborhood on Sunday, and government warplanes streaking overhead amid heavy bombardment of the area. Hundreds of IS fighters and allied militants are holed up in Hajar al-Aswad and the nearby Yarmouk Palestinian refugee camp.

Residents of Damascus reported hearing loud booms throughout the night and Sunday morning.

President Bashar Assad has escalated his military campaign to retake all remaining enclaves in the capital and surrounding areas. The IS-held areas in southern Damascus are the last holdouts, after rebels evacuated the eastern Ghouta suburbs following a fierce government offensive and an alleged poison gas attack in the town of Douma.

Chemical weapons inspectors collected samples from Douma on Saturday, two weeks after the suspected gas attack there prompted retaliatory strikes by Western powers on the Syrian government’s chemical facilities.

The site visit, confirmed by the Organization for the Prohibition of Chemical Weapons, will allow the agency to proceed with an independent investigation to determine what chemicals, if any, were used in the April 7 attack that medical workers said killed more than 40 people. The OPCW mission is not mandated to apportion to blame for the attack.

Douma was the final target of the government’s sweeping campaign to seize back control of eastern Ghouta from rebels after seven years of revolt. Militants gave up the town days after the alleged attack.

The Syrian government and its ally Russia denied responsibility for the suspected chemical attack.

Meanwhile, rebels have begun evacuating three towns in the eastern Qalamoun region in the Damascus countryside.

Al-Ikhbariya TV said that 35 buses left the towns of Ruhaiba, Jayroud, and al-Nasriya on Saturday carrying hundreds of rebels and their families to opposition-held territory in northern Syria.

The station said the evacuations would continue for three days.