A U.N.-brokered plan to stop the bloodshed in Syria effectively collapsed on Sunday after President Bashar Assad's government raised new, last-minute demands that the country's largest rebel group swiftly rejected.

The truce plan, devised by U.N.-Arab League envoy Kofi Annan, was supposed to go into effect on Tuesday, with a withdrawal of Syrian forces from population centers, followed within 48 hours by a cease-fire by both sides in the uprising against four decades of repressive rule by the Assad family.

But on Sunday, Syria's Foreign Ministry said that ahead of any troop pullback, the government needs written guarantees from opposition fighters that they will lay down their weapons.

The commander of the rebel Free Syrian Army, Riad al-Asaad, said that while his group is ready to abide by a truce, it does not recognize the regime "and for that reason we will not give guarantees."

Annan's spokesman had no comment on the setback. The envoy has not said what would happen if his deadlines were ignored.

Even before the setback, expectations were low that the Assad regime would honor the agreement.

Russia, an Assad ally that supports the cease-fire plan, may now be the only one able to salvage it. The rest of the international community, unwilling to contemplate military intervention, has little leverage over Syria.

In recent days, instead of preparing for a withdrawal, regime troops have stepped up shelling attacks on residential areas, killing dozens of civilians every day in what the opposition described as a frenzied rush to gain ground. Activists said at least 21 people were killed in violence on Sunday and as many as 40.

"Mortar rounds are falling like rain," said activist Tarek Badrakhan, describing an assault in the central city of Homs on Sunday. He spoke via Skype as explosions were heard in the background. The regime is exploiting the truce plan "to kill and commit massacres," he said.

Just as Annan complained Sunday that the escalation was 'unacceptable', Syria said its acceptance of the Annan deal last week was misunderstood and suggested it would not be able to withdraw its troops under current conditions.

In addition to demanding written guarantees from the opposition, Foreign Ministry spokesman Jihad Makdessi said Syria also wants assurances from Annan that Qatar, Turkey and Saudi Arabia - Assad's most active critics - halt "financing and arming of terrorist groups."

Qatar and Saudi Arabia are said to be creating a multimillion dollar fund to pay rebel fighters, while Turkey has floated the idea of creating buffer zones for refugees in Syrian territory, near the Turkish border.

Many had expected the Assad regime to stall and create new obstacles to a truce because it has little to fear from the international community, said Peter Harling, an analyst at the International Crisis Group think tank.

"Nothing seems to have a price tag," he said, noting that regime has been accused of shelling whole neighborhoods, exacting collective punishment and driving people out of their homes.

The regime might also be reluctant to move forward for fear of losing control.

While Annan's plan calls for eventual negotiations between the government and the opposition over Syria's political future, anti-regime activists say huge numbers of protesters would probably flood the streets and quickly topple Assad if he were forced to halt his yearlong crackdown.

Makdessi, the Syrian official, suggested that a truce without guarantees would give rebels the upper hand. He said Syria will not allow a repeat of what happened during the Arab League's observer mission in Syria in January, when Assad pulled back his forces, only to see rebels flood the vacated areas.

The Syrian foreign minister is expected in Moscow on Monday, but it is not clear whether Russia will step in to try to salvage the Annan plan it had supported enthusiastically.

Despite growing criticism of Assad, Russia has consistently shielded him from international condemnation.

Since the Syrian uprising erupted in March 2011, more than 9,000 people have been killed, the U.N. says.

On Sunday, Syrian forces pounded towns in the center and north of the country.

Activists said rebel fighters shot down a Syrian army helicopter with a heavy machine gun in northwestern Idlib province. The report came from the Britain-based Syrian Observatory for Human Rights and Idlib activist Fadi al-Yassin, both citing multiple witnesses. Al-Yassin said witnesses saw the helicopter crash to the ground, and that fighters were trying to make their way to the area.

Syria restricts access of foreign journalists, and activists' reports cannot be confirmed independently. There was no official comment.

Some of the heaviest fighting of the day was in Homs, where government troops attacked several rebel-held neighborhoods, said Badrakhan, the local activist.

In the Khaldiyeh neighborhood, 40 bodies were piled in a room in a makeshift hospital because the constant shelling has prevented burials, he said, adding that activists are aiming fans at the corpses so they won't decompose quickly.

"We might have to bury them in public gardens," he said.

Near the capital of Damascus, government troops raided the suburbs of Darya, Douma and Beit Jin.

The grassroots Local Coordination Committees put the day's death toll on the opposition side at 45, including six children. It said nine people were killed in Homs and 13 in Hama province, among them seven members of one family. The Observatory reported at least 21 civilians killed in fighting and shelling by government forces, along with seven rebel fighters and 12 soldiers.