The foreign ministry said it was "deeply shocked" by the killings and urged the swift implementation of UN-Arab envoy Kofi Annan's peace plan, a day after the United Nations condemned the Syrian regime's use of artillery in Houla.

But it stopped short of pointing the finger directly at the Syrian government, after Russia questioned whether Damascus was behind the violence.

"China is deeply shocked and strongly condemns the incident," said foreign ministry spokesman Liu Weimin.

"China... calls for an immediate investigation into this issue and to find the perpetrators. This incident again shows that Syria should waste no time to implement the ceasefire and end the violence," he added.

Beijing and Moscow -- both long-standing allies of Damascus -- drew international criticism earlier this year for vetoing two UN Security Council resolutions against President Bashar al-Assad's regime.

They have since backed Annan's efforts to bring peace to Syria. But the peace envoy's six-point blueprint, which was supposed to begin with a ceasefire from April 12, has been broken daily.

"We hope Annan will continue to play an active role and relevant parties will continue to provide support toward Annan's six-point proposal," Liu said.

"We urge relevant parties in Syria to immediately and comprehensively uphold relevant Security Council resolutions and Annan's six-point proposal, stop all violence, properly protect innocent civilians, ease tensions there and push forward the political resolution of the Syrian issue."

Syrian authorities have denied their forces carried out the Houla killings, which have sparked an international outcry, with a spokesman blaming "terrorists" and saying the government had opened an investigation.

On Monday, Russian Foreign Minister Sergei Lavrov said both sides in the conflict "clearly had a hand in the fact that peaceful citizens were killed" in Houla.

More than 13,000 people have been killed in Syria since an anti-regime revolt broke out in March 2011, according to the Syrian Observatory for Human Rights, a watchdog group.