HOMS, Syria (CBS/AP) -- The International Committee of the Red Cross and other relief agencies said Friday they were heading for the battered Baba Amr district of Homs, a day after government ground troops and tanks advanced on the residential area, forcing rebel fighters to pull out in what they claimed was a planned "tactical" retreat to reveal to the world the level of violence unleashed by President Bashar Assad on his own people.
"All relief agencies, including the Syrian Arab Red Crescent, left Damascus around 9 a.m. on their way to Homs. They are almost there," Damascus-based ICRC spokesman Saleh Dabbakeh told CBS News.
"They are loaded with humanitarian aid and medicine to provide urgent aid to civilians trapped by the fighting and facilitate the evacuation of the wounded," added Dabbakeh.
A Red Cross spokesperson said bad weather and heavy snow in the region was hampering the journey, but the convoy was expected to arrive by early Friday afternoon.
It was unclear how many civilians remained in Baba Amr on Friday, with reports putting the population still inside the district - which has been without food, water or power supplies for weeks amid the intense government assault - at anywhere from 10,000, to virtually nobody.
Video posted online by opposition activists showed residents apparently trying to collect falling snow to melt as drinking water.
Rebel fighters abandoned Baba Amr, a stronghold of the opposition since the uprising began almost a year ago, on Thursday. Activists said Assad's army was hunting down and assassinating handfuls of insurgents who stayed to cover their comrades' "tactical retreat."
The United Nations' human rights office expressed alarm Friday at the reports that summary executions were taking place after the Syrian government forces retook Baba Amr.
A spokesman for the U.N. High Commissioner for Human Rights said the global body had received unconfirmed reports of "a particularly grisly set of summary executions" involving 17 people in Baba Amr.
British Prime Minister David Cameron said Friday the Syrian government will pay for the violence it's committing against its own people, reports CBS News correspondent Larry Miller.
"What I think matters is building the evidence," Cameron said, "so that we hold this criminal regime to account. . . . One day, no matter how long it takes, there will be a day of reckoning for this dreadful regime."
Reports from Homs could not be verified due to tight government restrictions on media operations in Syria, and the level of danger in Baba Amr specifically being so great that few Western journalists have been there in recent weeks.
The relief agencies were also to transfer the bodies of two Western journalists killed in Homs by a government rocket strike last month to Damascus for forensic examinations and to begin the repatriation process, Syrian government officials told CBS News, though the Red Cross has said they are still unclear what the situation in Baba Amr is, and could not confirm plans to evacuate the remains.
Activists showed video Thursday of the badly burned body of Marie Colvin, a U.S. war correspondent who worked for the U.K.'s Sunday Times, before she was apparently buried in a temporary grave in Baba Amr. French photographer Remi Ochlik was also killed in the strike on a makeshift media center in Baba Amr.
The successful evacuation yesterday of two French journalists, Edith Bouvier and William Daniels, means all four surviving Western journalists who remained in Homs after the bombing of the media center have been successfully spirited out of the country.
Homs is about 12 miles northeast of the border with Lebanon, and cross-border smuggling has been key to the city's survival and to arming the rebels because of the links between Sunnis in northern Lebanon and the Sunni majority in Homs.
The United Nations said Syrian security forces have killed more than 7,500 civilians since the revolt began last March. Syria's government said in December that "armed terrorists" had killed more than 2,000 soldiers and police during the unrest.
Assad, a London-trained eye doctor, is increasingly isolated in his struggle to crush a vocal opposition movement which formally announced Thursday that it was militarizing with a move to unite disparate armed rebel groups under the Free Syrian Army - comprised largely of defected Syrian soldiers.