Fears grew of brutal reprisals by regime forces in the Baba Amr district of the battered Syrian city of Homs after a Red Cross relief mission to the vanquished rebel stronghold was blocked.
With world outrage growing against the Syrian regime, a monitoring group reported at least 38 Syrians killed Friday, including 10 shot dead in Baba Amr, the rebel district overrun by regime forces on Thursday.
After what was called a "tactical withdrawal," the opposition Syrian National Council (SNC) warned of "barbaric" reprisals against residents.
The UN rights body appealed to Syria to respect international law after receiving unconfirmed reports of 17 "grisly" executions as regime forces took control of Baba Amr.
Rupert Colville, spokesman for the Office of the High Commissioner for Human Rights, said information received suggested "a particularly grisly set of summary executions" on Thursday.
The Syrian Observatory for Human Rights said 10 people were shot dead in Baba Amr on Friday, but its head, Rami Abdel Rahman, added: "The circumstances of their deaths are not clear."
Meanwhile wounded British photographer Paul Conroy, who was smuggled out of Homs through Lebanon, said from a hospital bed in Britain that the regime's month-long bombardment of the city was "an indiscriminate massacre" and there were thousands of people there just "waiting to die."
French journalists Edith Bouvier, 31, and photographer William Daniels, 34, who arrived in Paris Friday after also being smuggled out of Syria, said the regime appeared to have "directly targetted" foreign media in Baba Amr in a wave of attacks which killed two of their colleagues.
On the ground, thousands of people poured onto the streets of Damascus and the coastal city of Aleppo Friday to urge the West to supply the rebel fighters with arms, prompting the security forces to open fire, said activists and monitors.
In New York, UN leader Ban Ki-moon demanded that Syria unconditionally let in humanitarian aid.
"The Syrian authorities must open without any preconditions to humanitarian communities," Ban told a press briefing at the UN headquarters.
"It is totally unacceptable, intolerable. How as a human being can you bear... this situation," he said.
But Syria's UN envoy Bashar Jaafari accused the UN chief of "slandering" President Bashar al-Assad's government with his accounts of the deadly crackdown on opposition protests.
The United States called all countries to condemn the "horrific" brutality in Syria as President Barack Obama declared that Assad's days were numbered.
For his part, French President Nicolas Sarkozy said that "what is going on is scandalous. There are more than 8,000 dead, hundreds of children, and the city of Homs faces the risk of being wiped off the map."
British Prime Minister David Cameron urged Syrians "butchering" their compatriots to turn their backs on the "criminal" Damascus regime or face justice for the blood on their hands.
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He described the situation as a "scene of medieval barbarity."
Syria has refused to let UN humanitarian chief Valerie Amos into the country, but Ban said the United Nations still hoped to persuade Assad to allow her entry to make an aid assessment.
His comments came after the Red Cross said Syria had blocked an aid convoy from entering Baba Amr, which was overrun by regime forces on Thursday after rebel fighters retreated in the face of their withering assault.
International Committee of the Red Cross president Jakob Kellenberger said the ICRC and the Syrian Arab Red Crescent Society were not allowed to enter Baba Amr more than a day after getting permission to do so.
"It is unacceptable that people who have been in need of emergency assistance for weeks have still not received any help," Kellenberger said in a statement in Geneva.
More than 20,000 civilians are believed to have been trapped in Baba Amr during a month-long bombardment by regime forces, with a lone doctor reported to be tending to the scores of casualties in a single makeshift clinic.
Volunteers on the ground said the Syrian army had prevented a convoy of seven trucks carrying food, medicines, blankets, baby milk and other supplies from going in.
One said they had been told this was because the army needed to "clear the sector of land mines and explosives left behind by rebels when they fled" on Thursday in the face of overwhelmingly superior regime fire power.
Photographer Conroy, 47, painted a bleak picture of Homs, which was relentlessly shelled for almost a month before this week's ground assault which routed the rebels.
"It's not a war, it's a massacre, an indiscriminate massacre of men, women and children," he told Sky News television.
He described the humanitarian situation as "more than a catastrophe", saying there was no power or water, and food was scarce.
"There's still thousands of people in Homs... they're living in bombed-out wrecks, children six to a bed, rooms full of people waiting to die," he said.
Another 12 people, including five children, were killed as a shell crashed into a crowd of demonstrators in Rastan, near Homs, monitors said, with 16 further deaths reported elsewhere in the country.
As the situation in Baba Amr remained dire, activists had called for nationwide protests to demand the arming of the rebel Free Syrian Army (FSA) to give it the firepower to defend itself against regime forces.
"Assad, don't delude yourself, there are a thousand and one Baba Amrs," the "Syrian Revolution 2011" Facebook page said..
France announced it was closing its embassy in Damascus, mirroring similar moves by Britain and the United States.
The ICRC spokesman in Damascus, Saleh Dabbakeh, said the remains of American reporter Marie Colvin and French photographer Remi Ochlik, the two Western journalists killed in the February 22 rocket attack that wounded Bouvier and Williams, were taken from Homs to the capital on Friday.