The Red Cross delivered emergency aid to areas around the battered Baba Amro district of the Syrian city of Homs toda, but was blocked for a third day from entering the former rebel bastion amid reports of bloody reprisals by state forces.
Activists reported shelling and other violence across Syria, sending one of the biggest surges of refugees across the border into Lebanon in a single day since a revolt against president Bashar al-Assad began a year ago.
Concerns mounted for civilians left stranded in Baba Amro in freezing weather with little food, fuel or medicine. Rebels abandoned their positions there on Thursday after facing almost a month of near-constant shelling by Syrian forces intent on crushing the uprising.
Activists said the government was trying to prevent the Red Cross from witnessing "massacres" by Syrian soldiers hunting down and killing remaining rebels.
"In all the years that I have known the ICRC in Damascus, the Syrian government has never let them see torture victims or the underbelly of the government," said Joshua Landis, a Syria expert at Oklahoma University. "I don't know why they would change now."
UN secretary-general Ban Ki-moon said on Friday he had received "grisly reports" that troops were executing and torturing people in Homs after insurgents abandoned their positions.
"It's over for tonight. We will try again tomorrow," said Saleh Dabbakeh, the Damascus-based spokesman for the International Committee of the Red Cross (ICRC), referring to efforts to get into Baba Amro. He declined to say why Syrian forces had blocked their entry.
The ICRC said workers had instead delivered supplies to areas nearby where many people had taken refuge.
The United Nations' refugee agency said up to 2,000 Syrians had fled the fighting across the border into Lebanon.
"We had similar numbers in April 2011, but the flow of new arrivals had stabilised since then," United Nations refugee agency spokeswoman Dana Sleiman said.
Refugees spoke of army shelling and gunfire in border towns. One woman said she and her family had fled the village of Jusiyah, near Qusair, about 12km from the border. "In the morning the shelling started, so we had to leave toward Lebanon. There were some wounded, but I don't know what happened to them," said Um Ali (64).
She sat under a tree with her husband, five sons and a pregnant daughter-in-law. They had not brought any belongings. "We don't know what to do," she said.
A doctor on the Lebanese border said around 1,500 Syrians had crossed into Lebanon.
A Lebanese security source said the Lebanese army had arrested at least 35 men armed with guns who were trying to enter Lebanon from Qusair. He did not give any indication of their allegiance.
The outside world has proved powerless to halt the killing in Syria, where repression of initially peaceful pro-democracy protests sparked an insurrection by army deserters and others.
The government says it is fighting foreign-backed "terrorists" it blames for killing hundreds of soldiers and police across the country.
Syrian state television showed images of the corpses of anti-Assad fighters killed in clashes in the suburbs of the city of Hama, as well as an array of captured weapons, including arms it said were US and Israeli-made.
Reuters