By Mohamed Misto

EASTERN GHOUTA, Syria

At least 250 civilians have reportedly been killed in Syria’s regime-besieged Eastern Ghouta district, a suburb of Damascus, within the last three days.

Since Tuesday, regime forces have stepped up attacks on Eastern Ghouta using barrel bombs, artillery fire and other types of weapons, according to an Anadolu Agency correspondent based in the area.

Over a three-day period, the regime has reportedly carried out 260 separate attacks in Eastern Ghouta, killing more than 250 civilian residents of the district.

Regime forces have struck 22 of the district’s health centers. On Wednesday night, they also targeted the Al-Nur Mosque in Eastern Ghouta’s Douma city with barrel bombs.

Regime attacks were reportedly still ongoing as of Thursday.

At peace talks held in Kazakh capital Astana in May of last year, Turkey, Russia and Iran designated Eastern Ghouta as a “de-escalation zone” in which acts of aggression would be expressly prohibited.

Nevertheless, Russia -- one of the agreement’s three guarantor-states -- has failed to prevent the Assad regime from repeatedly violating the terms of the truce.

The U.S. warned that Russia and the Syrian regime's actions "are on notice" amid the bombardment.

“Bashar al-Assad has already committed war crimes," White House spokesman Raj Shah told reporters. "He’s already gassed his own people with Sarin. He’s already committed unthinkable acts and he’s done so with Russian support. And we don’t want that to continue."

Saudi Arabia, meanwhile, has voiced its “deep concern” over the Syrian regime's continued attacks on the district and their impact on the civilian population.

On Thursday, the Saudi Press Agency quoted an unnamed Saudi official who called on the Syrian regime to allow badly-needed humanitarian aid into the district.

The Assad regime, the source said, “must seriously pursue a political solution [to the conflict] based on agreed-upon principles laid out in the Geneva 1 Declaration and UN Security Council Resolution 2254”.

Adopted by the UNSC unanimously in late 2015, the resolution calls for a lasting cease-fire and a viable political settlement in war-torn Syria.

Home to some 400,000 civilian residents, Eastern Ghouta has remained under a crippling regime siege for the last five years, which has brought the district to the verge of humanitarian catastrophe.

Syria has been locked in a devastating conflict since early 2011 when the Assad regime cracked down on demonstrators with unexpected ferocity.

According to UN officials, hundreds of thousands of people have been killed in the conflict to date.

*Michael Hernandez contributed to this report from Washington, D.C.