Syrian regime launches widespread shelling, opposition says Three men are killed by government forces Wednesday, an opposition group says Kofi Annan: If a U.N. monitoring mission fails to bring peace, Syria could plunge into civil war At least 23 people are reported dead Tuesday, an opposition group says (CNN) -- With the prospect of a full-scale civil war on the horizon, intense shelling pummeled cities across Syria on Wednesday, opposition activists said, prolonging a deadly year-long nightmare that shows no signs of ending. The fresh attacks by government forces took place in Hama, Daraa, Idlib and Homs provinces, said the Local Coordination Committees of Syria, an opposition network. Two men were killed by the shelling, and one was executed by security forces in Homs province, the LCC said. Despite months of international sanctions, diplomacy and pressure on President Bashar al-Assad's regime, world leaders concede an end to the slaughter may still be far away. Special envoy Kofi Annan told the U.N. Security Council on Tuesday that international observers are the last hope for success of a peace plan in Syria. Otherwise, he warned, Syria is sure to plunge into full-blown civil war. "The implication of that is quite frightening," Annan told reporters after briefing the Security Council. We cannot allow that to happen. Annan, the U.N.-Arab League envoy to Syria, plans to return to Damascus soon to seek adherence to a cease-fire that was supposed to go into effect April 12. He reiterated that the killing must stop immediately. "There has been some decrease in the military activities, but there are still serious violations in the cessation of violence that was agreed," Annan said. Mark Lyall Grant, the British ambassador to the United Nations, said Tuesday that 60 observers were on the ground, along with a staff of 37 civilians. He said the United Nations hoped to have 100 observers by the end of this week and 300 by the end of May. Susan Rice, the U.S. ambassador to the United Nations, said the United States stood behind its position that al-Assad has lost all legitimacy and must step down. If the violence persists, she said, the Obama administration is ready to look at other means to ratchet up pressure on Damascus, including a renewed discussion in the Security Council. On Monday, U.N. Secretary-General Ban Ki-moon said the Syrian situation has become one of the "most serious and gravest concerns of the international community." This is totally unacceptable and an intolerable situation," Ban said. Not long after his remarks, another 23 people were died across Syria on Tuesday, the LCC said. The Syrian ambassador to the United Nations again blamed the violence on armed perpetrators that he claimed were supported and financed by Turkey, Saudi Arabia, Qatar and other nations. "We are still committed toward guaranteeing the maximum success to the mission of Kofi Annan," Bashar Jaafari said at a news conference Tuesday. But the Syrian government cannot do all the job alone. He said the international community was acting hypocritically by not acting against terrorists in Syria as they do against al Qaeda. Rice said no one could say with certainty that there are no foreign fighters in Syria. But she said that's not the key issue. "This is substantially a diversion from the main point," Rice said. The main point is that the government of Syria continues to kill its own people. Many nations, including many of Syria's Arab neighbors, have condemned the ongoing violence, which has pitted a minority Alawite-dominated government against a predominantly Sunni uprising. International leaders have said the Syrian government is targeting dissidents seeking democracy and the ouster of al-Assad, whose family has ruled Syria for 42 years. The LCC says more than 11,000 people have been killed in 14 months. CNN's Joe Vaccarello, Amir Ahmed and Holly Yan contributed to this report.