Umit Bektas/Reuters Protesters waved Syrian and Turkish flags during a protest against Syria's president, Bashar al-Assad, at a refugee camp in Hatay province on the Turkish-Syrian border on Tuesday. BEIRUT, Lebanon - The cease-fire plan for the 13-month-old Syria conflict veered closer to collapse on Tuesday, with the Syrian government making new demands, the opposition ridiculing them as unrealistic, and no indication that Syria's military forces had withdrawn from major cities and towns before a midnight deadline as promised. The plan's chief architect, Kofi Annan, the special representative of both the United Nations and the Arab League, insisted the agreement was yet achievable. He appeared to lay much of the responsibility for the setbacks on the Syrian government in a letter submitted to the Security Council. The plan calls for the Syrian military to withdraw from major cities and towns before 12:01 a.m. Wednesday local time, but "in the last five days it has become clear that such a signal has yet to be issued," Mr. Annan wrote. In the letter, which was shared by diplomats in advance of its public release, Mr. Annan said Syrian officials had assured him that military forces had been withdrawn from some areas. Nonetheless, he wrote, "credible reports indicate that during that same period, the Syrian armed forces have conducted rolling military operations in population centers, characterized by troop movements into towns supported by artillery fire." Mr. Annan wrote that the Syrian government's surprise new conditions made in the past three days - guarantees of compliance from the opposition and instant deployment of international cease-fire monitors pre-approved by Damascus - "puts at risk the cessation of violence in all its forms that is so urgently needed." Earlier Tuesday, Mr. Annan told a news conference while visiting Syrian refugees in Turkey: "I believe it's a bit too early to say that the plan has failed. The plan is still on the table. He still held out hope for the second and more important deadline - 6 a.m. on Thursday for all combatants in the conflict to stop fighting. "It's a plan the Syrians have endorsed and from the comments made by the opposition, they're also prepared to go along with it if the government meets its commitments to pull the troops out," he told reporters. Mr. Annan's letter corroborated all indications on the ground that President Bashar al-Assad of Syria had no intention of abiding by the original terms of the cease-fire to halt the conflict, in which more than 9,000 people have been killed and many thousands wounded and displaced since it began in March 2011. The Syrian National Council, the umbrella group of anti-Assad exiles, said Mr. Assad's government had not only failed to comply with the cease-fire but that the new conditions it has demanded were "unacceptable and unrealistic." A spokesman for the council, Basma Qoudmani, told reporters during a visit to Geneva that "we have received a very strong message from the regime that it intends to intensify rather than beginning to comply with the cease-fire." Even Mr. Assad's most important foreign supporter, Russia, expressed a hint of exasperation with the Syrian government's behavior. "We expressed to our Syrian colleagues our view that their steps could be more active, more decisive in executing the relevant conditions of the plan," Foreign Minister Sergey V. Lavrov of Russia said in describing a meeting with his visiting Syrian counterpart, Walid al-Moallem, according to the Interfax news service. But Mr. Lavrov also criticized Syria's opposition, asserting that "it's common knowledge that the proposals of Kofi Annan have not been accepted by some if not the majority of opposition groups, including the Syrian National Council." And in a rejoinder to the United States, which has harshly criticized both Russia and China for thwarting more aggressive action on Syria at the Security Council, Mr. Lavrov said: "The U.S. and other countries, which have pretty direct access to Syrian opposition groups, would do better not to constantly blame Russia and China, but to use their levers in order to honestly force everyone to stop shooting each other." Neil MacFarquhar reported from Beirut, and Rick Gladstone from New York. Reporting was contributed by, Sebnem Arsu from Ankara, Turkey, and Alan Cowell from London. Hala Droubi from Beirut and Ellen Barry from Moscow.