A Syrian air raid killed at least 30 people in a rebel-held northern border town on Wednesday, opposition activists said, and a bomb went off near UN and military sites in the capital Damascus, wounding three.
As the violence intensified, UN human rights investigators accused forces loyal to Syrian President Bashar al-Assad of committing war crimes and crimes against humanity.
They said rebels had also committed war crimes, but the violations "did not reach the gravity, frequency and scale" of those by state forces and the pro-Assad shabbiha militia.
"The commission found reasonable grounds to believe that government forces and the shabbiha had committed the crimes against humanity of murder and of torture, war crimes and gross violations of international human rights law and international humanitarian law, including unlawful killing, torture, arbitrary arrest and detention, sexual violence, indiscriminate attack, pillaging and destruction of property," said the 102-page report by the independent investigators led by Paulo Pinheiro.
Residents in the northern border town of Azaz screamed and shouted "God is greatest" as they carried bloodied bodies from collapsed concrete buildings, video posted by activists showed.
The opposition Syrian Observatory for Human Rights said dozens had been killed. One activist in the town said at least 30 bodies had been found and rescuers were searching for more.
Video footage, which could not be verified, showed crowds of residents wrestling with steel bars and pulling away a giant slab of concrete to reveal the dust-covered arm of a child.
"This is a real catastrophe," said an activist who gave his name only as Anwar.
"An entire street was destroyed."
Assad's forces have increasingly used helicopter gunships and warplanes against the lightly-armed insurgents.
In Damascus, a bomb exploded in the car park of a hotel used by U.N. monitors, but several military buildings are also in the vicinity and it was not clear what the target was.
"Criminal and barbaric"
No UN staff were hurt in the blast, which occurred exactly four weeks after a bomb killed four of Assad's senior aides. The bomb set a fuel tanker ablaze and black smoke billowed over the city. Ash and dust covered white UN vehicles parked nearby.
State media said three people were wounded in the bombing and several rebels were killed or captured in a separate gunbattle with security forces in the western district of Mezze.
Deputy foreign minister Faisal Mekdad said the bomb blast proved "the criminal and barbaric nature of those who carry out these attacks - and their backers in Syria and abroad".
UN emergency relief coordinator Valerie Amos, on a mission to seek more access for aid deliveries, was meeting European Union officials in Damascus when the bomb went off.
She herself was unable to reach the town of Douma, a trouble spot just north of the capital, due to bombardment.
"Waiting at checkpoint to get into Duma. Sounds of shelling. Could not enter," Amos tweeted. The authorities told her she had been turned back for her own safety, her spokesman said later.
Groups calling themselves The Descendants of the Prophet Brigade and the al-Habib al-Mustafa Brigade said on a Facebook page they were jointly responsible for the bombing and that the attack had killed 50 soldiers. It was impossible to verify that claim.
Last month Assad's troops successfully counter-attacked after rebels seized parts of Damascus. They are still trying to dislodge insurgents from Aleppo, Syria's biggest city.
A Syrian air strike has wrecked a hospital in a rebel-held area of Aleppo, a doctor there said on Wednesday, an attack that New York-based Human Rights Watch said violated international law.
At least two holes gaped in the walls of Al Shifaa Hospital and four floors were heavily damaged by Tuesday's raid.
"If we had lingered just another five minutes, we would have died," said the surgeon, who gave his name only as Younes.
Dust covered hospital beds, incubators were broken and the floor was strewn with rubble. Water from a broken tank had leaked out, mixing with patches of blood.
More than 50 were killed across Syria on Wednesday, the Syrian Observatory said, after 160 the day before.
Opposition sources say 18,000 people have been killed since the uprising against Assad erupted in March last year.
Syrian state media said Amos met Foreign Minister Walid al-Moualem to discuss the growing needs of civilians affected by the "destruction of private and state property by terrorist armed groups" - the government's usual term for rebels.
The bloodshed has divided regional and world powers, foiling peace efforts and paralysing the U.N. Security Council on Syria.
Muslim heads of state were expected to suspend Syria from the Organisation of Islamic Cooperation at a summit in Mecca on Wednesday, over the objections of Iran, Assad's closest ally.
The 57-member body's rebuke is mostly symbolic, but it shows Syria's isolation in much of the Sunni-majority Islamic world.
Syria's own Sunni majority is the dynamo of the revolt against Assad, whose Shi'ite-rooted Alawite minority is at the core of a ruling system based on the army and security services.
Syria has been caught up in a wider sectarian-tinged tussle pitting Shi'ite Iran against Saudi Arabia and its Sunni-ruled Gulf allies.
Turkey has also turned against Assad. Tehran is determined to prop up a proven ally who has provided vital support for its Shi'ite ally Hezbollah in Lebanon.
Ankara and Washington have pledged to step up aid to the Syrian opposition and planning for a post-Assad Syria.
Secretary of State Hillary Clinton said on Saturday the United States and Turkey were looking at all ways to help Syrian rebel forces, possibly including a no-fly zone.
US officials later emphasised that no such step toward direct military intervention was likely any time soon.
"Of course we should evaluate these issues. However, our discussions of these issues with Turkey should not suggest we are making commitments to set up these zones," US ambassador to Ankara Frank Ricciardone was quoted as saying.
A Nato-led no-fly zone and bombing campaign helped Libyan rebels overthrow Muammar Gaddafi last year. But the West has shied away from an overt military role in Syria.
In a sign of how the war in Syria affects its neighbours, a Shi'ite clan in Lebanon said it had kidnapped more than 20 Syrians there after rebels seized a kinsman in Damascus.
Clan member Maher al-Meqdad said the action was to win the release of Hassan al-Meqdad, held for the past two days by Free Syrian Army rebels who said Hezbollah had sent him to Syria.
Maher al-Meqdad said his relative had gone to Syria before the uprising began and had no links to the fighting there.
Syria's uprising has polarised Lebanon, where Sunnis mostly support the rebels, while Hezbollah backs Assad.