"The regime will not implement this plan. This plan will fail," Free Syrian Army chief Riad al-Asaad told Reuters.
Escalating violence has already raised questions over the ceasefire. Opposition activists said dozens of people were killed and wounded on Sunday when President Bashar Assad's loyalists shelled a rebellious area near the border with Turkey.
UN-Arab League envoy Kofi Annan, seeking to end a conflict that has killed more than 9,000 people in the past year, said the latest bloodshed violated the guarantees he had been given and urged Damascus to keep its promises.
The deal Annan brokered calls on Syria to begin the pullback of troops from around towns and cities by Tuesday and for a truce to start 48 hours later.
While emphasizing that would happen, Foreign Ministry spokesman Jihad Makdissi said in a statement that Syria also wanted the written guarantees.
"Syria has a plan for military pullback already in place and being implemented, but completing and achieving the main goal would definitely require the guarantees from the other side and those supporting them to abide by the terms of calm," he said.
"(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," he said.
Syria also sought guarantees that Qatar, Saudi Arabia and Turkey -- outspoken in criticizing Assad -- would not fund the armed groups.
Annan made no specific reference to the new Syrian demands in a statement from his office in Geneva.
He expressed shock at the "surge in violence and atrocities." Each side has accused the other of intensifying assaults in the run-up to the truce.
"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," he said.
FSA leader Asaad said his group had not been asked to deliver written guarantees to end violence.
"We have given our word that if the regime commits to the plan then we will too," he said. "We are honest."
"Nobody has asked us for anything written. Nobody has discussed with us handing over our weapons. We will never hand over our weapons."
Colonel Qassem Saad al-Deen, spokesman for the joint command of the FSA inside Syria, said the rebels would respect the deadline to cease fire.
"We will commit to the deadline even if they (government forces) do not pull back, we will cease fire as we have pledged to the UN," he said.
"But if they fire we will pick up arms again and fight them," he told Reuters from inside Syria.
"When the regime asks Kofi Annan for written guarantees that we will drop our weapons it is actually mocking the United Nations. This is a joke."
He said at least 1,000 people had been killed during last week's violence in Syria, most of them civilians.
The Syrian government says its opponents have killed more than 2,500 troops and police since the unrest began in March 2011.Army offensive
Dozens of people were killed and wounded on Sunday when Assad's tanks shelled an area in the rebellious province of Idlib, near the border with Turkey, opposition activists said.
Fighters from the rebel Free Syrian Army were surrounded in the village of al-Bashiriya, activists said.
"The army is shelling al-Rouge with tanks, and helicopters are firing rockets at al-Bashiriya. Tens of people have fallen dead or injured but we cannot get to them because the bombardment is heavy," said activist Mahmoud Ali, with the sound of helicopters audible on the phone.
The Syrian Observatory for Human Rights said at least 21 people were killed on Sunday in shooting and shelling in Homs, Deraa, Idlib, Deir al-Zor and Hama province, and that security forces had detained at least 200 in raids across the country.
It said that separately, at least 12 government soldiers were killed in clashes.
The Syrian army has launched an offensive to seize back large swathes of Idlib that had fallen under rebel control. Thousands of Syrians have fled to Turkey.
No comment on the fighting was immediately available from Syrian officials. The government has placed tight restrictions on media access, making it hard to verify witness accounts.
A team led by Norwegian Major-General Robert Mood has been in Damascus since Thursday discussing how around 250 UN monitors might oversee a ceasefire.
Annan said he was in constant contact with the Syrian government and urged "all states with influence on the parties" to exert it now to help ensure an end to the bloodshed and the beginning of a political dialogue.
The former UN Secretary-General plans to go to Tehran on April 11 for talks with senior Iranian officials. Iran is a key ally of Syria.
BEIRUT (Reuters) -- Syria on Sunday demanded written guarantees insurgents will stop fighting before it pulls back troops under the terms of a UN peace plan, and a rebel leader said the initiative was doomed.