The International Red Cross says Syrian authorities have denied it access to the devastated Baba Amr district of Homs for a second day.
A spokesman said they were negotiating and would not give up as the need on the ground was great.
Syrian officials say the area is being cleared of booby traps.
But there have been reports of revenge killings by Syrian forces since rebel fighters pulled out, and shelling has been reported in other areas of Homs.
Meanwhile, the bodies of a US journalist and a French photographer, who were killed in Homs, have left Damascus on a flight to Paris.
"I confirm that... the bodies of Remi Ochlick and Marie Colvin are aboard the Air France flight which has just left Damascus bound for Paris" on Saturday evening, France's ambassador to Damascus Eric Chevallier told the AFP agency.
Their bodies had been handed over to diplomats earlier at the Al-Assad University Hospital in Damascus, reports said.
'Street by street'
The seven-lorry Red Cross aid convoy spent the night in Homs and is set to spend a second after being blocked from entering Baba Amr, despite having been initially given permission from the government.
"[The International Committee of the Red Cross and Syrian Arab Red Cross Society] could not enter Baba Amr today," said the UK spokesman of the ICRC, Sean Maguire.
"Negotiations to gain access to the suburb continue. Our teams remain in Homs, ready to enter Baba Amr as soon as possible."
But he said their presence in Baba Amr was vital.
"The needs on the ground are quite large and we need to get in quickly," he said, adding that they were not giving up.
"We haven't turned back... We're persistent."
The BBC's Jim Muir, in neighbouring Lebanon, says the reason being given by the Syrian authorities is that there are mines and potential booby traps in Baba Amr that need to be cleared first.
But he says there have been unconfirmed reports of revenge killings and summary executions by Syrian forces in Baba Amr and opposition activists believe the delay is to cover this up.
The reports speak of mass arrests of males over the age of 11, with the local cooperative building being turned into a detention centre.
One report alleged that a lorryload of dead bodies from Baba Amr was seen on a nearby highway. There were also reports of explosions and shootings in other nearby districts to which many families from Baba Amr had fled.
There were reports of renewed shelling in other parts of Homs on Saturday.
The Syrian Network for Human Rights group said mortar and machine-gun rounds had been fired into Jobar, adjacent to Baba Amr, while the Local Coordination Committees network said the districts of Khaldiya, Bab Sbaa and Khader had also been shelled.
Syrian state media said there had also been a suicide car bomb attack in the southern city of Daraa that had killed two people.
Syrian state television has broadcast pictures from inside Baba Amr that show massive destruction, which it blamed on "armed terrorist gangs" carrying out a foreign plot to undermine Syrian stability.
Conditions in Baba Amr are said to be terrible, with no power and little food, water and medical supplies.
Our correspondent says the TV pictures showed nobody at all on the streets and that until the Red Cross gains access to make an assessment it will be impossible to say how many people remain.
One activist, Bassel Fouad, who has escaped to Lebanon, said colleagues who stayed behind had told him that pro-government gunmen were "entering homes and setting them on fire".
"They begin at the start of a street and enter and search house after house,'' he told Associated Press. "Then they start with another street.''
In an address to the UN General Assembly on Friday, Secretary-General Ban Ki-moon said the international community had failed in its duty, and inaction had encouraged Syria's leaders in their repression of civilians.
Mr Ban said it was time for the international community to speak with one voice.
"Continued division emboldens the Syrian authorities in their violent path," he said.
He added: "The disproportionate use of force by Syrian authorities has driven what had been largely peaceful opposition forces to resort to take up arms in some cases."
Meanwhile Paul Conroy, a Sunday Times photographer who fled Syria after being wounded in Homs, told the BBC that what was happening in Baba Amr was "systematic slaughter".
Mr Conroy, who was smuggled out of Syria into Lebanon on Tuesday, described the scenes in Homs from his hospital bed in the UK.
"I've done a fair few wars, I've never seen anything on this level," he said.
"There are no targets, it's pure systematic slaughter of a civilian population."