Syria's Bashar Assad regime and Iran-backed terror groups attacked a center used by the White Helmets civil defense agency in northwestern Idlib Sunday, killing a civil defense worker, according to local sources.

The attack was carried out in Marek district, which is located inside the Idlib de-escalation zone.

The civil defense center was heavily damaged and became unusable as a result of the attack, the sources said on condition of anonymity due to restrictions on speaking to the media.

Meanwhile, at least four civilians were killed in an airstrike in Syria's northwestern Idlib province on Saturday.

Mustafa Haj Yousuf, head of the White Helmet civil-defense organization, told Anadolu Agency (AA) that a warplane targeted the civil-defense team which was conducting search and rescue efforts in Idlib's Jisr al-Shughur district where was hit by an earlier attack.

Four people, including a civil-defense volunteer, were killed, while two other civil-defense volunteers were seriously injured, Haj Yousuf said.

The SU-34 Russian warplane took off from Khmeimim air base, according to a statement by the opposition.

The Bashar Assad regime and its Iranian-backed terror groups have continued heavy bombardment in residential areas of Idlib since early Saturday.

The warplanes of the Assad regime also started airstrikes in the evening hours. Regime attacks in Idlib's de-escalation zone are estimated to have killed at least 116 civilians, and injured more than 342 others, since the beginning of 2019.

The Sochi agreement was reached on Sept. 17 by President Recep Tayyip Erdoğan and his Russian counterpart Vladimir Putin. According to the agreement, the cease-fire in the Idlib region will be preserved, with the withdrawal of heavy arms and radicals from the region. Prior to the agreement, the Bashar Assad regime was signaling a grand operation on Idlib, which is the last stronghold of the opposition, sparking deep fears in the international community of a new humanitarian crisis. As Turkey and Russia stepped in and averted a possible disaster, the Sochi deal was internationally welcomed. In relation to the implementation of the agreement, Turkish and Russian officials have been taking positive steps, stressing that the process has been continuing in line with the deal.

Yet, the regime and its supporters have been attempting to violate the agreement. Since the Sochi agreement in September 2018, more than 35 civilians have lost their lives in attacks by the regime, with many others injured.