International envoy Kofi Annan urged the Syrian government on Tuesday to withdraw its troops and halt all violence in 48 hours, saying its military operations and new conditions to implement his peace plan are putting a desperately needed ceasefire at risk.

In a letter to the U.N. Security Council, obtained by The Associated Press, Annan said President Bashar Assad's government has not pulled troops and heavy military equipment out of cities and towns -- as his six-point plan called for by Tuesday.

Annan, the joint U.N. and Arab League envoy, said it is essential that the Syrian government "seize the opportunity to make a fundamental change of course" in its military operations and halt all violence by 6 a.m. local time on April 12, the deadline set in his plan.

"It is essential that the next 48 hours bring visible signs of immediate and indisputable change in the military posture of the government forces throughout the country, as called upon by the six-point plan," Annan said.

In a letter read to council members Tuesday at a closed meeting, Annan said opposition members have indicated that they would observe a ceasefire "provided Syrian forces withdraw from cities."

He said the days before Tuesday's deadline for Syria to withdraw all troops and heavy military equipment from cities and towns should have been an opportunity for the government "to send a powerful political signal of peace" with action on his peace plan.

He said the absence of U.N. observers made it difficult to fully ascertain the situation on the ground but "the rapid increase of refugee flows into Turkey gives a strong indication of a surge in violence."

Annan said in a letter on April 8 that the Syrian government introduced new conditions for full implementation of the plan:

-- "Written assurances that armed opposition groups are prepared to cease all violence."

-- "Immediate disarmament of armed groups."

-- "Commitment by regional countries that they would not finance or arm opposition groups."

Annan said these new "ex-post facto requirements" were not part of the six-point plan the Assad government agreed to on March 25.

"This puts at risk the cessation of violence in all its forms that is so urgently needed and which must take place on April 12 at 6 a.m." local time, he said.

Annan said he had been advised by Russia's Foreign Minister Sergey Lavrov following his meeting in Moscow with Syrian Foreign Minister Walid Moallem "that the Syrian government is no longer insisting on written guarantees, but would need me to assure that the other parties and governments also accept the plan."

Annan said his team has been in close contact with "the whole spectrum of the opposition" including the leadership of the Syrian National Council, the Free Syrian Army, local coordination committees and other groups, to explain what is required of them alongside the government's obligations.

Annan sent a technical team to Damascus to discuss arrangements for the possible deployment of U.N. monitors for any cease-fire between Syrian troops and rebel forces. The team has been envisioning a deployment of some 200-250 unarmed monitors.

Annan told the Security Council that "progress on technical issues appeared to have been made but substantive differences emerged" and the team leader was returning to Geneva for consultations.