GENEVA (Reuters) - Syrian government troops and affiliated forces committed war crimes and a crime against humanity in their long siege of eastern Ghouta through heavy bombardment and “deliberately starving” 265,000 people, U.N. investigators said on Wednesday.

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About 20,000 rebel fighters entrenched in the besieged area, some belonging to “terrorist groups”, shelled the nearby capital of Damascus in attacks that killed and maimed, amounting to war crimes, they said.

The latest report by the U.N. Commission of Inquiry on Syria, led by Paulo Pinheiro, is based on 140 interviews as well as photographs, videos, satellite imagery, and medical records.

“Even if pro-Government forces are bombing and starving the civilian population of eastern Ghouta into submission, there can be no justification for the indiscriminate shelling of civilian inhabited areas in Damascus,” panel member Hanny Megally said in a statement.

Pro-government forces encircled Ghouta in April 2013 and “laid the longest running siege in modern history, steadily wearing down both fighters and civilians alike through a prolonged war of attrition”, the report said, denouncing a “medieval form of warfare”.

From February to April 2018, their tactics to recapture the enclave were “largely unlawful in nature, aimed at punishing the inhabitants of eastern Ghouta and forcing the population, collectively, to surrender or starve”.

“Through the widespread and systematic bombardments of civilian inhabited areas and objects, and the continued denial of food and medicine to besieged civilians during the period under review, pro-Government forces perpetrated the crime against humanity of inhumane acts causing serious mental and physical suffering,” the report said.

Syrian and Russian air forces controlled the skies over Ghouta and carried out heavy air strikes on residential areas where families huddled in dark, overcrowded basements lacking toilets, it said.

Warplanes hit hospitals, depriving wounded of health care, it said, adding: “This pattern of attack strongly suggests that pro-Government forces systematically targeted medical facilities, repeatedly committing the war crime of deliberately attacking protected objects, and intentionally attacking medical personnel.”

The U.N. experts cited evidence that chlorine had been used in Ghouta at least four times this year but said their investigations continued.

Armed groups including Jaish al-Islam, the region’s largest, Faylaq al-Rahman, Ahrar al-Sham and Hay’at Tahrir al-Sham exercised control over Ghouta.

They operated tunnels and deep trenches, and regularly arrested and tortured members of religious minority groups, the report said.

Government attacks were “regularly met with brutal acts of reprisal”, it said, saying the rebels’ use of improvised rockets that could reach parts of Damascus aimed to “spread terror”.

By mid-March, pro-government forces implemented negotiated local truces and “evacuation agreements”, the report said. Tens of thousands left via humanitarian corridors amid chaos and many civilians remain “interned”.