Anti-regime protests were expected across Syria on Friday despite a week of violence that has killed dozens of people and severely tested a UN-backed peace accord.
Demonstrations were to take place after the weekly Muslim prayers in flashpoint cities including central Hama, where shelling by government troops has reportedly killed more than 100 people since Monday.
President Bashar al-Assad's regime has blamed "armed terrorist groups" for the violence, and for failing to abide by a ceasefire that took effect more than two weeks ago.
But UN chief Ban Ki-moon said Syria was in contravention of a six-point peace deal by keeping troops and heavy weapons in urban areas, while expressing alarm about reports population centres were shelled.
Opposition figure Anwar al-Bunni said the accord drawn up by UN-Arab League envoy Kofi Annan was likely to fail because it obliges Syria to allow free demonstrations.
"If the Annan plan which provides for peaceful demonstrations is applied, millions of Syrians will take to the streets and the regime will fall," the Cairo-based rights activist told AFP.
"It is clear the Syrian regime cannot implement the Annan plan," said Bunni, adding the violence carried out by pro-government forces "is planned".
Violence across Syria on Thursday killed 22 people, including 16 civilians who died at the hands of government forces, and six regime loyalists killed in fighting in Aleppo, the country's second-largest city, said the Syrian Observatory for Human Rights.
Overnight clashes between troops and rebels in the central city of Homs killed at least one army deserter and wounded another 15, it said in a statement.
More than 9,000 people have died since a popular uprising erupted against Assad's regime in March 2011, the UN says, while non-governmental groups put the figure at more than 11,100.
The truce, which has barely held since taking effect on April 12, is to be monitored by 300 UN observers due to arrive in Syria in coming weeks. A small advance team is already on the ground.
Ban "remains deeply troubled by the continued presence of heavy weapons, military equipment and army personnel in population centres, as reported by United Nations Military Observers," said a UN statement.
This was "in contravention of the Syrian government's commitments to withdraw its troops and heavy weapons from these areas," he said, demanding Damascus "comply with its commitments without delay."
Western nations have expressed strong doubts that the UN observers will be able to work. The United States has already warned it may not renew the mission's initial three-month mandate.
US ambassador to the United Nations Susan Rice said the Security Council must be ready to order sanctions if Syria flouts commitments to halt violence.
"We condemn what remains the government's refusal to abide by its commitments, its continued intense use of heavy weaponry in Hama and elsewhere, which continues to result in large numbers of civilian deaths every day," she told reporters.
The Syrian National Council, the main opposition group in exile, has called for an emergency Security Council meeting to discuss a resolution to protect civilians.
"Hama in recent days, and following a visit by UN observers, witnessed a series of crimes... that left more than 100 people dead and hundreds wounded because of heavy shelling," it said.
Among those killed were nine activists "summarily executed" after meeting UN monitors, a rights group said.
The reports cannot be verified because of restrictions on foreign media.
The Arab League said it would ask the UN to ensure the immediate protection of civilians, but without going as far as demanding the use of force.
"The entire world is waiting for a truce and the observers to be deployed, but unfortunately the fighting has not stopped and every day new victims die," Arab League chief Nabil al-Arabi said after a meeting in Cairo.
On Wednesday, France raised the prospect of military intervention if Annan's peace plan fails.
Foreign Minister Alain Juppe said the international community would have to move to a Chapter Seven resolution, which authorises foreign powers to take measures including military options.
However, Juppe added such a resolution, which was also mooted by US Secretary of State Hillary Clinton last week, was unlikely to pass, alluding to previous Security Council vetoes by Russia and China.
Moscow, a long-time Damascus ally, blamed the recent violence in Hama on rebel forces and hinted at Al-Qaeda involvement.
"Opposition groups have essentially reverted to waging wide-scale terror in the region," said foreign ministry spokesman Alexander Lukashevich.
Attacks aimed at "killing as many peaceful civilians as possible and destroying civilian infrastructure remind one of what is happening in Iraq, Jordan and other places where Al-Qaeda and its groups operate," he said.