BEIRUT — A weekend massacre of more than 100 people emerged as a potential turning point in the Syrian crisis Monday, galvanizing even staunch ally Russia to take an unusually hard line against President Bashar Assad’s government.

Analysts said Russia may be warning Assad that he needs to change course or lose Moscow’s support, which has been a key layer of protection for the Syrian government during the uprising that began in March 2011.

Russia has grown increasingly critical of Damascus in recent months, but Foreign Minister Sergey Lavrov’s latest comments were unusually strong. Although he said opposition forces have terrorists among them, he put the blame for 15 months of carnage primarily on Assad’s government.

“The government bears the main responsibility for what is going on,” Lavrov said in Moscow after a meeting with British Foreign Secretary William Hague. “Any government in any country bears responsibility for the security of its citizens.”

Alexei Malashenko, a Middle East expert with the Carnegie Moscow Center, said Lavrov’s comments suggest Russia may be backing away from its long-standing support for Damascus.

“Bashar Assad is driving himself and Russia into a corner,” Malashenko said. “Bashar has definitely gotten the sense that he may lose Russia’s sympathy, and he may step back a bit.”

It is not clear whether Assad’s forces were exclusively to blame for the slaughter of 108 people Friday in Houla, a collection of poor farming villages in Homs province. The United Nations said 49 children and 34 women were among the dead, and that some were shot in the head.

The U.N. Security Council blamed Syrian forces for artillery and tank shelling of residential areas, but it did not clearly state who was responsible for the close-range shooting deaths.

Former U.N. Secretary-General Kofi Annan arrived Monday in Damascus to try to rescue an already faltering peace plan that was set back further by the massacre.

Annan said he was “personally shocked and horrified by the tragic incident in Houla” and declared that “a critical moment” had arrived.

“I urge the government to take bold steps to signal that it is serious in its intentions to resolve this crisis peacefully,” said Annan.

Russia blamed both the government and the rebels for the Houla massacre. “Both sides have obviously had a hand in the deaths of innocent people, including several dozen women and children,” Lavrov said. “This area is controlled by the rebels, but it is also surrounded by the government troops.”

He said Russia has no interest in propping up Assad, but wants Syria to guide its own transition under a plan brokered by Annan.

“We don’t support the Syrian government; we support Kofi Annan’s plan,” Lavrov said.