Syrian troops pummeled opposition neighborhoods in the city of Homs with heavy mortars Wednesday after peace envoy Kofi Annan said Damascus had again promised to respect an international peace plan. “Annan, this is your ceasefire,” ran the sarcastic voiceover on an activist video that showed a shopping mall engulfed in flames after it was hit in bombardment of the Juret Al-Shayah quarter. Sniper fire cracked out incessantly in the background. At least 12 people were killed Wednesday, activists said. Western powers have scorned President Bashar Al-Assad's truce pledges to the United Nations-Arab League mediator, but so far lack an effective policy to curb the bloodshed, given their own aversion to military intervention and the resistance of Russia and China to any UN Security Council action. “Far from fulfilling their commitment, the regime has cynically exploited the window of diplomatic negotiations to crack down even harder on its own people,” British Prime Minister David Cameron said during a visit to Indonesia. He said Annan had reported that Assad's troops were “conducting roving military operations in population centers supported by artillery fire”. An estimated 1,000 people had been killed in the past week, Cameron said. With all hostilities with rebels supposed to end at dawn Thursday, activist videos posted on YouTube showed bombs crashing into the Khalidiya district of Homs. Spouts of pulverized debris burst high into the air with each impact and plumes of dust and smoke drifted over the rooftops. The videos could not be verified and the Syrian government bars most independent media from the country. The Syrian Observatory for Human Rights (SOHR), a British-based activist information organization, said three people were killed in shelling on Homs that went on for an hour. “Mortar fire started this morning. I can hear one explosion every five minutes,” said activist Waleed Al-Fares in Homs, where bombardment killed at least 26 people on Tuesday. China expressed “deep worries” about the violence in Syria and called for all sides to respect a ceasefire. Foreign Ministry spokesman Liu Weimin also described Annan's work as “an important and rare opportunity” to end the crisis.