Syrian fighter jets blasted the rebel-held town of Maaret al-Numan on Thursday, killing at least 49 people including 23 children, rescuers said.
UN peace envoy Lakhdar Brahimi pressed for a truce during a key Muslim holiday later this month, while the UN's human rights chief appealed to the Security Council for unity over the crisis.
Rescuers said bombs destroyed two residential buildings and a mosque, where many women and children had been taking refuge, in the strategic northwestern town. Among those killed was a nine-month-old baby.
Rebels captured the town on October 9 in a push to create a buffer zone along the Turkish border.
"We have recovered 44 corpses from under the rubble," one worker told an AFP correspondent at the scene.
In a makeshift field hospital, the correspondent saw at least 32 bodies wrapped in white sheets, including six children and many mutilated corpses, as well as plastic bags marked "body parts."
One child was decapitated, the correspondent said. The body of a second was pulled from the rubble still on his bicycle.
"At the moment it seems only three people survived the attack, including a two-year-old child," said medic Jaffar Sharhoub. "He survived in the arms of his dead father."
A resident who spoke to AFP on condition of anonymity said several of those killed had just returned to their homes. "They thought the danger had passed," the resident said.
Several fighter jets flew over Maaret al-Numan and the surrounding area throughout Thursday morning.
They made short dives to drop at least 10 bombs on the town and its eastern outskirts, near the besieged Wadi Deif army base, which came under heavy bombardment by the rebels.
Later Thursday, another five people were killed during the bombardment of the town, said rebels.
In the early evening, the rebels launched what they said was a "final assault" on the base, a key depot for tanks and fuel supplies.
Hundreds of fighters attacked the base, a frontline AFP correspondent reported. Three tanks were destroyed and at least six soldiers surrendered, rebel officers said.
The base is situated two kilometres (a mile) from the Damascus-Aleppo highway, of which the rebels control a stretch of several kilometres. That has severely impaired the army's ability to resupply units under fire in the northern metropolis for the past three months.
In Damascus, a suicide bomber on a motorcycle blew himself up just 300 metres (yards) from the interior ministry without causing any casualties, a security source said.
The bombing came ahead of Brahimi's arrival in Damascus to press his call for a ceasefire during the four-day Eid al-Adha Muslim holiday starting on October 26.
Speaking in neighbouring Jordan, the UN-Arab League envoy said he hoped that such a temporary ceasefire could form the basis for a longer lasting truce. The alternative would be disastrous for the whole region, he warned.
"If the ceasefire is implemented, we can build on it and make it a real truce as well as the start of a political process that would help the Syrians solve their problems and rebuild their country," Brahimi said in Amman.
"If the Syrian crisis continues, it will not remain inside Syria. It will affect the entire region," he added.
Brahimi is due in the Syrian capital on Friday afternoon and will hold talks the following day with Foreign Minister Walid Muallem. It will be his last stop on a tour of countries that play influential roles in the crisis -- Egypt, Turkey, Saudi Arabia, Iran, Iraq, Lebanon and Jordan.
The envoy will also meet with President Bashar al-Assad "very, very soon, but not on Saturday," Brahimi's spokesman, Ahmad Fawzi, told AFP.
Asked whether any optimism was warranted over the Damascus talks, foreign ministry spokesman Jihad Maqdisi said: "Let's see what Brahimi has to say."
Serious doubts have been raised about Brahimi's plan to halt the bloodshed, even temporarily.
"I don't know whether they will all agree at the higher level or not on the ceasefire proposal, but on the ground you have pro- and anti-regime forces that do not respond to any authority," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
Damascus said it is ready to discuss with Brahimi his proposal, but wanted assurances that countries with influence on the rebels would pressure them to reciprocate.
The exiled opposition said it would welcome any ceasefire but insisted it was for the government to halt its daily bombardments.
With nearly 200 people killed on Thursday across the country according to the Syrian Observatory, the death toll since March last year came to more than 34,000.
In Geneva, the UN's human rights chief Navi Pillay appealed for the Security Council to settle their differences over Syria.
"The situation in Syria is quite simply dire," said the High Commissioner for Human Rights.
"I urge the Security Council to speak with one voice. This is essential in order to send a strong message," she added.
She repeated that violations committed by both sides "may amount to war crimes or crimes against humanity."
Moscow and Beijing, two of the five permanent members of the UN Security Council, have so far blocked three draft resolutions on Syria backed by Western and Arab countries, accusing them of interference in Syrian affairs.