UNITED NATIONS (AP) — The UN peacekeeping chief said Tuesday there are strong suspicions that pro-regime fighters were responsible for some of the 108 victims of a massacre in Syria, along with heavy weapons illegally fired by Syrian government forces.

Herve Ladsous told reporters he has seen no reason to believe that “third elements” — outside forces — were involved in one of the bloodiest single events in Syria’s 15-month-old uprising, though he did not rule it out, saying “we may learn more.”

The Syrian government has denied any responsibility for the massacre in Houla, blaming “armed terrorists.”

Ladsous strongly disagreed.

“I am certainly saying that because people — civilians, children — were dramatically killed by heavy weapons, I am saying for this, undoubtedly, the government of Syria is responsible,” he said.

Ladsous pointed the finger at pro-government militias known as shabiha as well. “There were strong suspicious that the shabihas were involved in this tragedy in Houla,” he said.

But he added, “I cannot say that we have absolute proof.”

Ladsous also pointed to the involvement of the shabiha in other attacks.

“There have been similarly very strong rumors in other places, in other instances of extreme violence, they were involved,” Ladsous said, without giving any examples.

“When you look at the situation from the ground,” he said, “you see a number, of course, military and security forces who are in uniform, but you see also a substantial number of people who are dressed in civilian clothes yet are heavily armed with machine guns and all that.”

Ladsous, who visited Syria last week, said he didn’t ask those armed civilian if they were shabihas, but “the fact is that some elements are definitely present on the scenes of violence.”

The peacekeeping chief said almost all of the 300 unarmed UN observers authorized by the Security Council are now on the ground. They are deployed in eight cities and will soon be in 11 cities, he said.

Ladsous said the presence of UN observers is “saving lives” and the use of heavy weapons has diminished in areas where they are deployed.

But Assistant Secretary-General Tony Banbury, who deals with equipment and logistics, said the armored vehicles they use for patrols “are being shot at on an almost daily basis,” including on Tuesday.

Asked who was to blame, Ladsous said “when bullets are fired into an armored car there’s no signature on the bullets.”

“You cannot establish with absolute certainty which side is responsible,” he said. “But I do believe that over the series of incidents both sides have had some responsibility.”

Ladsous stressed that the goal is to implement all six points in international envoy Kofi Annan’s peace plan, starting with a halt to violence and withdrawal of heavy weapons and troops from populated areas and ending with peace talks to settle the conflict. He said the government must also allow access to detainees and peaceful demonstrations.

Ladsous and Annan’s deputy, Jean-Marie Guehenno, are scheduled to brief a closed Security Council meeting on Wednesday.

Britain’s UN Ambassador Mark Lyall Grant told reporters that, during closed council consultations on the Mideast on Tuesday, he warned “that the current situation was not sustainable” and that the Houla massacre “is seen by many in the region, and more widely, as a game-changer.”

Britain wants council members to hold a strategic discussion at Tuesday’s meeting on next steps in Syria, Lyall Grant said.

He said he expects Annan, the joint UN-Arab League envoy who is currently in Damascus, to brief the council in the next few days, after he returns to Geneva.

White House: No military intervention

Meanwhile, the White House said it remains opposed to military action in Syria, reasoning that it would only lead to more carnage.

Spokesman Jay Carney said the Obama administration will continue offering non-lethal assistance to the Syrian people. Following a gruesome weekend massacre in Syria, Carney said the US is working with its allies to assess further action, but he gave no specific detail about what those next steps might include.

A United Nations report said more than half of the 108 people killed over the weekend were women and children.

Carney reaffirmed US calls for Syrian President Bashar Assad to leave power. Carney also said Tuesday’s coordinated move by the US and other nations to expel Syrian diplomats was a signal of the international community’s “absolute disgust” with Assad’s rule.