The prospects of a ceasefire in Syria suffered a jolt on Sunday after the Syrian government sought from Kofi Annan, the United Nations envoy, written guarantees that the armed opposition would not use force when Syrian forces were withdrawing from areas of conflict to meet an April 10 deadline.

According to Mr. Annan’s plan, ceasefire will come into force 48 hours after the Syrian forces completed their pull-out. Mr. Annan’s plan also envisaged the uninterrupted flow of humanitarian aid to the affected people, as well as the beginning of an all-Syrian political process to complete the country’s political transition.

In a statement, Syrian Foreign Ministry spokesman, Jihad Makdissi, said that the impression that Damascus “confirmed that it will pull back its troops from and around cities on April 10 is a wrong explanation”.

He clarified that Mr. Annan “has not delivered until now written guarantees regarding the approval of terrorist armed groups to end violence and readiness to lay down its weapons”. Besides, Syria had also sought guarantees that committed the governments of Qatar, Saudi Arabia and Turkey against funding the armed groups.

Syria has repeatedly alleged foreign intervention in its raging internal conflict, which according to U.N. count has already left more than 9,000 people dead. The Syrian statement explained that it was seeking guarantees because of its experience in the past when it pulled back forces during the presence of observers from the Arab League, only to find that armed opposition groups had regrouped and fanned out nationwide, in a number of districts.

As the April 10 deadline loomed, Syria appears to have stepped up operations to weaken remaining rebel strongholds. Unconfirmed reports — mostly because of the absence of independent journalists on the ground — say that Syrian forces, using tanks and helicopters, have assaulted the al-Rouge Plain southwest of Idlib city, close to the border with Turkey.

The heavy surge in fighting prompted a sharp rebuke from Mr. Annan, who said on Sunday that the situation was “unacceptable” as it deviated from the promises made by the government to end violence.

“As we get closer to the Tuesday 10 April deadline, I remind the Syrian government of the need for full implementation of its commitments and stress that the present escalation of violence is unacceptable,” Mr. Annan said.

“I am in constant contact with the Syrian government,” he said, “and ask all states with influence on the parties to use it now to ensure an end to the bloodshed and the beginning of dialogue.”

In preparing for a successful ceasefire, Mr. Annan appeared to be racing against time. In a video conference from Geneva with the United Nations General Assembly, he talked about the need to prepare a special type of peacekeeping force for Syria, which had a “broad and flexible mandate”.

“What we would need on the ground is a small nimble United Nations presence,” he said. The former U.N. Secretary-General added that such a force should engage all Syrian parties and “constantly and rapidly observe, establish and assess facts and conditions on the ground in an objective manner”. The new force must have a “unique” character because violence in Syria “cannot be averted through the means of the traditional observer mission interposed between two armies”.

On Thursday, the Syrian government welcomed Major-General Robert Mood, a Norwegian national, as part of an advance U.N. team to assess various aspects related to the deployment of peacekeepers.

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