The United Nations’ special envoy for Syria, Kofi Annan, arrived in the Syrian capital Monday in an attempt to shore up an already shaky U.N. peace plan following the massacre of at least 100 civilians in a central Syrian village.

Annan arrived amid fresh reports of violence in the city of Hama and deepening international concern about the killings Friday in the village of Houla, near Homs, in which at least 49 children died, according to the United Nations.

Annan said he was “personally shocked and horrified” by the deaths in Houla, which the U.N. Security Council blamed on the Syrian government in a sharply worded statement Sunday.

“This was an appalling crime, and the Security Council has rightly condemned it,” Annan said in a statement issued on his arrival in Damascus. He said he expected to hold “serious and frank” discussions with President Bashar al-Assad and called on him to “comprehensively” implement the six-point peace plan mandated by the United Nations in April. “This is not happening,” he said, in a blunt acknowledgment that the plan has not stemmed the bloodshed.

The massacre was one of the worst single incidents of the 14-month-old uprising against Assad’s rule, in which more than 10,000 people are believed to have died, and has heightened concerns that Syria is sliding into an unstoppable conflict that could spread beyond its borders.

Activists reported Monday that at least 34 people died in shelling by government forces overnight in Hama.

In Houla, at least some of the 108 people who the U.N. observer mission says were killed had been shot dead, and some of the bodies bore signs of physical abuse, according to U.N. Secretary General Ban Ki-moon. Diplomats at the United Nations said they had been told that as many as 116 were dead.

According to Houla residents, many of the killings were carried out by pro-government militias belonging to Assad’s minority Alawite sect; they went house to house in the Sunni village killing entire families overnight Friday.

The Syrian government has denied responsibility and accused the opposition of carrying out the killings.

In Washington, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, warned Monday that such “atrocities” could trigger international military intervention. He stressed, however, that he would like to see the global community exert greater diplomatic pressure on Assad before exploring military options.

“You’ll always find military leaders to be somewhat cautious about the use of force, because we’re never entirely sure what comes out on the other side,” he said on Fox News Channel. “That said, it may come to a point with Syria because of the atrocities.”

Sunday’s Security Council statement blamed most of the deaths in Houla directly on “government artillery and tank shellings on a residential neighborhood” in violation of a Security Council resolution that calls on Syria to withdraw troops and tanks from residential areas.

The statement was the international body’s strongest condemnation yet of the Syrian government’s behavior, and it signaled the harshest criticism of Damascus to date by Russia and China, Syria’s allies.

But it contained no threat of new measures against the Syrian government, and the international community is too divided to come up with effective ways of putting pressure on the regime, said Salman Shaikh, director of the Brookings Doha Center. Notably, he said, Russia remains opposed to taking tougher action against its chief Mideast ally.

“This gruesome incident could give real momentum to diplomatic efforts, but so far it’s not had much effect,” Shaikh said. “There is still the hope among key Western powers that the Russians will come round. But I don’t think it’s really changing tack.”

The differences were apparent Monday during a visit by British Foreign Secretary William Hague to Moscow, where he and his Russian counterpart, Sergei Lavrov, asserted their support for Annan’s peace plan. Lavrov made it clear that Russia also holds armed rebels responsible for much of the violence that appears to have escalated in recent days.

“The guilt has to be determined objectively,” Lavrov told reporters. “No one is saying that the government is not guilty, and no one is saying that the armed militants are not guilty.”

Hague said the rebels also are at fault but stressed that he believes the government “has the primary responsibility” for the violence.