Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

Syrian activists and rebel fighters said Monday that at least 100 people, many of them civilians, have been killed in a five-day Syrian government offensive in a predominantly Sunni area of the Damascus countryside. The death toll could be the largest from a single military campaign in nearly a year.

Activists described a bloody war zone inside Jdeidet al-Fadel, an area west of the Syrian capital that remained critically isolated Monday, a day after government forces reportedly had withdrawn to the town’s outskirts, trapping the surviving residents inside and keeping medical workers and activists out.

“The regime does not allow anyone to get in or out of the area. There is no way to save the injured people,” said a rebel military spokesman who uses the pseudonym Musab Abu Qatada.

News of the possible massacre trickled out of the Damascus area on a day when tensions flared along Syria’s border with Lebanon, where the chances of spillover violence have become increasingly likely. Lebanon fought a gruesome sectarian war in the 1980s, and its government has sought to stay out of the Syrian conflict.

Syria’s predominantly Sunni rebels have accused the Lebanese Shiite militant group Hezbollah of backing Syrian government forces in heavy fighting around the border town of Qusair in recent weeks. And Monday, two prominent Lebanese Sunni clerics, Ahmad al-
Assir and Salem al-Rifai, called on Lebanese Sunnis to fight on behalf of the Syrian rebels against Hezbollah, threatening to upset Lebanon’s precarious sectarian balance.

The Washington Post’s Liz Sly and David Ignatius look back at the bloody Syrian civil war--thousands killed, a country in ruins and borders breached by a tide of refugees. What will the future hold for the Syrian people and the al-Assad regime, and how does the U.S. fit into that picture? (Jason Aldag/The Washington Post)

The Syrian Observatory for Human Rights, a Britain-based watchdog group, said Monday afternoon that activists in Jdeidet al-Fadel had documented 101 dead inside the town, including at least 10 women and three children. But the Observatory said in a statement that the actual death toll could exceed 250 because “there are missing people that are hard to reach while the regime forces are deployed to the town.”

Activists also said that a number of the area’s residents were arrested over the weekend as rebel fighters ran out of ammunition and retreated, allowing forces loyal to President Bashar al-Assad to accelerate their offensive. Some estimated that hundreds had disappeared; they also said that area shops were looted and that homes were burned.

“Those arrests were taking place in a street where the [rebel Free Syrian Army] was not present, and they could not stop it,” said Aya Mahaini, an activist who spoke by Skype, using an alias, from the neighboring suburb of Madhamiyat al-Sham. The suburb also has been shelled repeatedly in recent days. The sounds of explosions frequently interrupted the conversation.

It was impossible to verify specific events or the number of missing or casualties beyond the accounts of activists. The United Nations says that at least 70,000 people have been killed and nearly a quarter of Syria’s population has been displaced in more than two years of brutal civil war.

The Syrian Arab News Agency, a mouthpiece for the Assad government, said Monday that government forces had “inflicted heavy losses upon terrorists in the town of Jdeidet al-Fadel in Damascus Countryside, injuring a number of them [and] killing others.”

Assad’s government refers to rebel fighters as “terrorists.”

A pro-regime Facebook page titled “Syria 24” reported Monday that government forces in Jdeidet al-Fadel had killed “600 terrorists” from Jabhat al-Nusra, a rebel jihadist group that the U.S. government has labeled a terrorist organization for its ties to al-Qaeda.

Videos posted online Sunday and Monday purported to show rows of bodies from the beleaguered town. Some of the people appeared to have been shot in the face. In one video, a camera moved slowly over what looked like 10 bodies covered by blankets and body bags, as another person revealed the victims’ faces. At least two appeared to be children.

A distraught man off camera identified one of the victims as his friend, Ibrahim Salama. “Put some light on him. Let the whole world see,” he said, pulling back the blanket from the man’s face.

The Local Coordination Committee for Jdeidet Artuz, the area that encompasses Jdeidet al-Fadel, posted on its Facebook page a list of 101 casualties — including names, ages and causes of death — that the group said it had confirmed.

Among them were Hanan Aslan, 6, who reportedly was killed by a mortar shell; Khaled Ibrahim Salloum, 33, said to be felled by a sniper shot; and Akram Moussa, age unknown, who the site said was dismembered using machetes. The cause of many other deaths was listed as burning; the victims were adults, their identities unknown. Other entries said the victims were “executed at the hands of the regime in the field.”

Activists said the Jdeidet al-Fadel area is one of a few contested suburbs at risk of falling under opposition control, as rebel forces have advanced toward the capital in recent months. The region provides the government with a critical lifeline to the Lebanese border.

“The [rebel] battalions are trying to connect the liberated areas, to surround the regime and strangle it,” said Abu Qatada, the rebel military spokesman.

On Monday, Abu Qatada said the area was back under regime control.

Activists said Jdeidet al-Fadel is populated predominantly by Sunni refugees from the Golan Heights, Syrian territory that was seized by Israel in the 1967 war, as well as those recently displaced by fighting in the Damascus countryside.

The area had no Internet access and only a marginal rebel presence when the government forces pushed in, activists said Monday. So the flow of information out of the area has been limited.

“The FSA has no real presence inside the city,” Abu Qatada said, referring to the Free Syrian Army. He identified fighters who were active in the city in the past five days as poorly armed “sleeper cells,” backed by some FSA members from neighboring areas who were able to enter the suburb to help. “There were no battalions, just very small groups inside the city,” he said.

Syrian activists and rebels said the Jdeidet al-Fadel offensive appears to be the biggest massacre since a government assault last summer on the nearby Damascus suburb of Darayya, which activists say left at least 300 people dead.

On Monday, some activist groups cited death tolls in Jdeidet al-Fadel that exceeded 500. But the accounts did not include specific lists of names.

The disparate groups that make up Syria’s rebel movement have pleaded for weapons and aid from the West to bolster the arms shipments they have received from Persian Gulf nations, such as Qatar and Saudi Arabia, in recent months. On Sunday, Secretary of State John F. Kerry pledged to expedite shipments of new, nonlethal aid pledged by Washington over the weekend. The administration has sent $385 million in humanitarian aid through international organizations and has funneled $117 million in funds and supplies directly to Syrian opposition groups.

Kerry said Saturday at a conference of Syrian opposition leaders and their allies in Istanbul that the new aid would total $123 million.

Karen DeYoung in Istanbul and Suzan Haidamous in Beirut contributed to this report.