BEIRUT — Syria has blamed up to 800 rebel fighters for the massacre in Houla last week that killed more than 100 people.

A news conference Thursday by Qassem Jamal Ihsan, who headed the government’s investigation into the massacre, was the most detailed response by the regime since the massacre began late Friday.

The slaughter in Houla brought immediate worldwide condemnation of President Bashar Assad. The UN says there are strong suspicions that “shabiha” fighters, shadowy gunmen who operate on behalf of the regime, were responsible for at least some of the carnage.

But Ihsan categorically denied any regime role in the massacre, saying hundreds of rebel fighters carried out the slaughter after launching a coordinated attack on five security checkpoints.

In the wake of the Houla massacre, the Syrian National Council called on UN special envoy Kofi Annan on Thursday to increase the number of UN observers in Syria tenfold to 3,000.

“The effectiveness of the observer mission must be improved in the cities controlled by the Assad regime — specifically Homs, Rastal, Houla, and Hama,” the main opposition group said.

The 300 UN observers currently in the country are scheduled to remain in Syria until mid-July.