The Syrian government is "in contravention" of an internationally agreed peace plan by keeping troops and heavy weapons in cities, Ban Ki-moon, UN secretary-general, has said.
He also said on Thursday he was "gravely alarmed" by reports of shelling of populated areas in Syria, in a statement released a day after a deadly explosion in the flashpoint city of Hama.
Activists blamed the government for the blast and put the death toll at 69, including 16 children. Syrian state media said 16 people were killed when a bomb that "terrorists" were building exploded prematurely inside a house .
An unverified video posted on YouTube showed the collapsed remains of the building, as ash-covered men dug through piles of masonry looking for bodies amid the cries of onlookers.
Abu Ghazi, an activist and Hama resident, told Al Jazeera: "People are dying because we don't have the necessary medical equipment or expertise to save those injured."
According to reports, an unexplained explosion was heard by residents in Damascus on Friday following overnight fighting in Douma, a suburb of the Syrian capital.
Later on Friday, a second explosion was reported by Syrian state media in the central Damascus district of al-Midan.
Al Jazeera cannot independently verify accounts of violence due to restrictions imposed on international media.
More than 9,000 people have died since a revolt erupted against President Bashar al-Assad's government in March last year, the UN says, while non-government groups put the figure at more than 11,100.
Under strong international pressure, Assad's government has agreed a troop withdrawal from cities as part of a six-point peace plan for Syria agreed with Kofi Annan, UN-Arab League envoy.
|Activists said the blast in Hama was caused by government shelling or even a Scud missile attack
The truce, which officially started on April 12 but has barely held, 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", the UN statement said.
This was "in contravention of the Syrian government's commitments to withdraw its troops and heavy weapons from these areas," Ban said, while demanding that Assad's government "comply with its commitments without delay".
Meanwhile, Annan's spokesperson has told the Reuters news agency in Geneva that 15 more ceasefire monitors of a total advance team of 30 are expected to be in Syria by Monday.
"We expect the 30 will be on the ground by the end of April, on Monday," Ahmad Fawzi said. "There is no delay... It is a whole process... They are deploying at remarkable speed."
Al Jazeera's Rula Amin, reporting from Beirut in the capital of neighbouring Lebanon, said Annan "would like as many ears and eyes on the ground as possible".
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.
Susan Rice, the US ambassador to the UN, said on Thursday 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 said.
In a statement, Syria's main opposition group, the Syrian National Congress, said: "We are calling for an emergency meeting of the UN Security Council so that it can issue a resolution to protect civilians in Syria.
"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."
In recent violence, at least 40 people were also reportedly killed in Hama on Monday, including nine activists "summarily executed" after meeting UN monitors, a rights group said.
The reports cannot be verified because of restrictions on international media.
The Arab League, for its part, said in a statement it would ask the UN to ensure the immediate protection of civilians in Syria, without however going as far as demanding the use of force.
Nabil Elaraby, the Arab League secretary-general, called for observers to be deployed rapidly.
"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," he told a ministerial meeting of the regional bloc in Cairo on Thursday.
"The important thing now is the ceasefire, and this will only happen if a sufficient number of observers is deployed."