ANTAKYA, Turkey - United Nations Special Envoy to Syria Kofi Annan said Friday that he was "shocked and appalled" by opposition reports from a small central village that Syrian forces killed more than 200 people on Thursday.

If true, it would be the single bloodiest attack since the uprising began last March. It was impossible to verify opposition activists' claims that forces loyal to President Bashar al-Assad had shelled the village of Tremseh, near Hama, and then conducted a ground operation.

The head of the U.N. monitoring mission in Syria - which doesn't venture out because of the ongoing violence - said his monitors witnessed the use of artillery, tanks and helicopters from their base around four miles away. "If we have credible cessation of violence and a local ceasefire, we stand ready to go in with a larger team to verify the facts on the ground," said Gen. Robert Mood.

The British-based Syrian Observatory for Human Rights told ABC News that shelling started at 5 a.m. Thursday. That was followed by a ground operation carried out by security forces and troops. They said they had confirmed the deaths of more than 150 people.

Other activist accounts placed the death toll north of 200 and accused pro-regime "shabiha," or thugs, of going on a deadly rampage in the village following the military's attack. The attackers, they said, went into the Sunni village from surrounding villages that are Alawite, the Shiite sect Assad belongs to.

The account is similar to the attack on the village of Houla at the end of May that left 108 dead, around half of them children. The Assad regime agreed many were killed in Tremseh, but accused "tens of terrorists" who "ransacked, destroyed and burned scores of the village houses…"

They accused the terrorists - their catch-all term for opposition fighters - and "bloody media outlets" of taking advantage of a Thursday U.N. Security Council meeting on Syria to "manipulate public opinion against Syria."

On Friday, communications with Tremseh appeared to be cut, making it more difficult to confirm the accounts. Video of the aftermath was scarce; one clip showed just over a dozen men who had been killed while another purported to show a long row of bodies in white burial shrouds in a mosque.

The Wall Street Journal reported Friday that Syria has begun to move some of its chemical weapons from their secure locations, fueling fear they could be used. Syria is known to have one of the biggest chemical and biological weapons stockpiles in the Middle East, including sarin nerve agent, VX gas, mustard gas and cyanide.

"This could set the precedent of [weapons of mass destruction] being used under our watch," a U.S. official told the Wall Street Journal. "This is incredibly dangerous to our national security."

Aside from the fears of their use, there are also fears the weapons could fall into the wrong hands if the regime falls, whether it's the al Qaeda-linked elements currently fighting against the regime or Syrian allies Hezbollah and Iran.

"Our assessment remains what it's been for some time that the Syrian regime has control of its chemical weapons stockpiles," Pentagon spokesman George Little told reporters on Friday. "We are watching very closely, not just the United States, but the international community, to make sure that they maintain control over those stockpiles and of course…to ensure that they don't use them." He also warned that the regime using the weapons "would cross a serious red line."