GENEVA — The starvation and bombardment of Syrian civilians in a long-besieged Damascus suburb overshadowed efforts on Thursday by United Nations diplomats to inject life into another round of peace talks in Geneva aimed at ending the nearly seven-year-old war.

The special United Nations humanitarian adviser for Syria, Jan Egeland, expressed outrage over what he described as heavy casualties in the suburb, Eastern Ghouta, and the inability of aid workers to help the 400,000 residents trapped there.

“There has been massive loss of life — hundreds and hundreds have been wounded,” Mr. Egeland said, describing Eastern Ghouta as an unfolding “catastrophe.”

As fighting has declined in large parts of the country, Eastern Ghouta has become an epicenter of the conflict in recent weeks, largely severed from supplies of food and medicine and under constant bombing by the Syrian Air Force and shelling by pro-government and rebel ground forces.

Russia, the Syrian government’s most important ally, told the United Nations on Wednesday that it had arranged a two-day truce in the area, but the fighting has continued. “In general, there is no calm in this de-escalation zone. There is only escalation in this de-escalation zone,” Mr. Egeland told reporters.

Amnesty International reported Thursday that Syrian aircraft had attacked Eastern Ghouta with cluster munitions over the past 10 days, and accused the government of President Bashar al-Assad of “war crimes on an epic scale.”

The organization said at least 10 civilians had been killed by the cluster munitions, which are internationally banned weapons that disperse bomblets that fan out, killing and maiming indiscriminately.

Earlier in the month, doctors in Eastern Ghouta supported by the Syrian American Medical Society said they had treated people showing symptoms consistent with exposure to chemical agents, possibly phosphorous.

The United Nations has reported soaring rates of malnutrition among Syrian civilians in combat zones amid food prices that have spiraled upward.