BEIRUT — Syrian government forces defied a new U.N. Security Council resolution calling for a cease-fire Sunday by launching a ground offensive, sustaining their airstrikes and allegedly dropping at least one bomb laden with chlorine against a rebel-held enclave outside Damascus.

Residents of the Eastern Ghouta suburb of Damascus spent a seventh consecutive day huddled in shelters as warplanes and artillery pounded the area, killing at least 22 and dashing hopes that this latest in a long string of failed efforts to tamp down the relentless bloodshed in Syria would work.

Among the dead was a child who suffocated when a chlorine bomb was dropped on a neighborhood of the suburb, the first chemical attack since the negotiations for a cease-fire began this month, according to doctors in the area.

“It was one more terrible day in Syria,” said Panos Moumtzis, the U.N.’s regional coordinator for Syria, who initiated the cease-fire call three weeks ago. He said he was “very, very disappointed” by Sunday’s violence but hoped a way to secure at least a temporary truce could still be found.

The United States and its allies had hailed as a diplomatic victory the unanimous vote in New York on Saturday approving a 30-day cease-fire, which was expected to go into effect “without delay.”

But to secure the support of Russia, the language of the resolution was diluted to exclude unspecified “terrorists,” a loophole that seems to be providing justification for continued fighting on many fronts.

The Syrian government routinely describes all of its opponents as terrorists, even though only a small number of the armed groups fighting in Syria are designated as terrorists by the rest of the international community.

Apart from saying that the cease-fire should go into effect as soon as possible, the resolution provided no deadline, no mechanism for establishing a deadline, and no process for enforcing the truce should it take hold.

A top Iranian military official said that Iran and Syria would abide by the cease-fire resolution, but that the suburbs of Damascus would not be included because they are “under terrorists’ control.”

Iran is one of President Bashar al-Assad’s closest allies, and has provided much of the military support that has helped him prevail over the rebellion.

“The cleansing operation will continue in those areas,” said Maj. Gen. Mohammed Hossein Baqeri, chief of staff of the Iranian armed forces, according to Iran’s Tasnim news agency.