An explosion ripped through a poor neighborhood in the opposition stronghold of Hama this morning, disrupting the fragile Syrian ceasefire agreement brokered by U.N. envoy Kofi Annan last month.

The death toll — and who’s responsible for the carnage — are disputed.

Syria state media reported that 16 people were killed after “an explosive device went off while a terrorist group were setting it up in a house which was used to make explosives.” But opposition activists say 70 civilians were massacred when a row of cement shanty homes collapsed following intense government shelling.

“The regime is committing all sorts of violations to Annan’s plan and until this moment it has not abided by any of the plan’s points,” the Syrian National Council, a high-profile opposition group said in statement about today’s attack. “We call for an emergency session (at the United Nations) to issue a resolution to protect Syrian civilians.”

The British-based Syrian Observatory for Human Rights said it was unable to determine the cause of the explosion, and also called for the U.N. to investigate.

Last Saturday the U.N. authorized sending 300 additional monitors to Syria to ensure compliance with the ceasefire, but only a dozen have arrived in the country thus far. The attack and other recent reports of violence have further stoked fears that the tenuous ceasefire will collapse.

Opposition activists told The New York Times that “government forces had singled out Hama for punishment because of a series of antigovernment protests when the first United Nations monitors visited last Sunday.”

Resentment runs deep in Hama, where 30 years ago then-Syrian President Hafez al-Assad launched what’s known as one of the bloodiest chapters of modern Arab history: the Hama massacre.

Back then, the city of Hama was the stronghold of Syria’s Muslim Brotherhood and the center of an anti-regime uprising that had been targeting government buildings and minority Alawite military officers for years.

“In 1982, the regime basically said, ‘That’s it. That’s enough. We have to deal with this once and for all. We have to show that we’re in control,'” Syria expert David Lesch told FRONTLINE for our November film, The Regime, an excerpt of which is embedded below.

Estimates vary, but between 10,000 to 30,000 people are believed to have been killed or disappeared in the massacre, which also left parts of the city in shambles. “It looked like a war zone,” remembered Syrian scholar Amr Al Azm.

Today President Bashar al-Assad, faces a similar issue in Syria’s fourth-largest city, which has seen some of the country’s biggest protests, and some of its worst violence since the uprising broke out more than a year ago. Last summer the Syrian opposition gained control of the city for six weeks, until the government sent troops into the city in August for a brutal assault that killed more than 100 civilians within its first 24 hours alone.

“Hama looked like Cairo. It looked like Tunis. This was a popular uprising,” explained the late New York Times correspondent Anthony Shadid, who had been reporting from Hama when it was free from government control. “And the government was very threatened by that narrative.”