BEIRUT - A U.N.-brokered cease-fire for Syria appeared to collapse on Sunday, two days before it was to go into effect, as the government demanded new conditions that the main rebel group swiftly rejected.

Syrian Foreign Ministry spokesman Jihad Makdessi on Sunday placed a new condition -- that the opposition agree in writing "to halt violence with all its forms and their readiness to lay down weapons." He also alleged that Qatar, Saudi Arabia and Turkey are arming and funding the opposition and demanded guarantees that this will be halted.

The opposition and Western leaders had been skeptical all along that President Bashar Assad would live up to his commitment to a truce because he broke similar promises in the past and escalated attacks on opposition strongholds in the days leading up to the cease-fire deadline.

"Whenever the regime pulls out its troops and tanks, people will march to Damascus and topple the regime," said Damascus-based activist Maath al-Shami. "Frankly speaking, the regime will collapse if checkpoints are removed."

The government showed no sign of relenting in its offensive against the opposition Sunday, as its forces pounded opposition strongholds in northwestern and central Syria. The violence underlined the Damascus' predicament: Acceptance and implementation of the U.N. plan, which calls for a full cease-fire, risks spelling the end of an autocratic regime which has relied largely on force to stay in power over the past four decades.

The six-point deal, brokered by U.N. and Arab League envoy Kofi Annan, was due to take effect on Tuesday, paving the way for negotiations to end the country's year-old crisis, which the U.N. estimates has killed 9,000 people. It calls for government forces to withdraw from population centres by Tuesday, to be followed by a full cease-fire by both sides by 6 a.m. Thursday.

Annan's spokesman, Ahmad Fawzi, refused to comment on the new demands.

The commander of the rebel Free Syrian Army, Riad al-Asaad, said his group was prepared to abide by the Annan agreement, but rejected the government's new unilateral demand.

The FSA does not recognize the regime "and for that reason we will not give guarantees," he told The Associated Press by telephone from his base in neighbouring Turkey. He said government forces should return to their bases and remove checkpoints from the streets.

With the deadline looming, Syrian forces have stepped up attacks on restive towns in recent days, and activists say scores of civilians have been killed daily.

Annan condemned the government offensive, saying in a statement Sunday that "the present escalation of violence is unacceptable." He also reminded the government "of the need for full implementation of its commitments," though it was unclear whether his statement was issued before or after Damascus imposed new conditions for complying with the truce.

The cease-fire was meant to pave the way for negotiations between the government and the opposition to end the country's year-old crisis. Street protests against Assad erupted 13 months ago, inspired by the Arab Spring's pro-democracy uprisings in the region, but the revolt has turned violent in the face of a brutal regime crackdown.

The government stance was reminiscent of a failed mediation attempt by the Arab League around the start of the new year. Assad also agreed to that plan to pull tanks and artillery out of cities and allow in foreign monitors in to assess compliance. But the mission ended in failure and Assad ultimately did not comply with the terms of the agreement he had signed on to.

Makdessi, the Foreign Ministry spokesman, said Syria will not allow a repeat of what had happened in January, when Assad pulled back his armed forces from cities and their surroundings, only to see rebels flood the areas vacated by government troops.

"Armed terrorist groups used this to rearm its elements and spread its authority on entire districts," Makdessi said.

Some of the heaviest fighting Sunday was taking place in the central city of Homs, where government troops fired mortar rounds at the rebel-held neighbourhood of Khaldiyeh, said activist Tarek Badrakhan from Homs.

"Mortar rounds are falling like rain," Badrakhan told The Associated Press via Skype as explosions could be heard in the background. He said troops were attacking on three fronts the rebel-held areas Deir Baalbeh, Khaldiyeh and Old Homs.

"They are giving the regime permission to kill and commit massacres," said Badrakhan referring to the cease-fire. He added that people have not been able to bury the dead in Khaldiyeh because of the shelling. He said that 40 bodies are piled in a room in a makeshift hospital, and that activists are trying to keep the bodies cold by directing a fan toward them.

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

The Britain-based Syrian Observatory for Human Rights said intense clashes occurred in Homs' Qusour, where defectors were able to kill four government soldiers and damage an armoured vehicle.

In the northwest, the Observatory said regime troops were fighting near the town of Jisr al-Shughour. The group also reported clashes between troops and defectors in several towns in the northern province of Aleppo.

Activists also reported raids in the Damascus suburbs of Darya, Douma and Beit Jin.

The Local Coordination Committees said 23 people were killed Sunday, including five in Beit Jin. The Observatory said clashes and government troops shelling killed at least 14 civilians as well as seven defectors and 12 soldiers.

At the Vatican, Pope Benedict XVI implored Damascus to heed international demands to end the bloodshed.

"Particularly in Syria, may there be an end to bloodshed and an immediate commitment to the path of respect, dialogue and reconciliation, as called for by the international community," Benedict said in his traditional Easter speech.