United Nations humanitarian chief Valerie Amos has visited Baba Amr in Homs, becoming the first international official to enter the embattled Syrian district since fighting began.
Amos' spokesperson told the AFP news agency the city was "completely devastated".
According to reports Amos also heard gunfire during her brief visit to the district.
Her visit is expected to result in the first outside assessment of conditions in an area of Homs city which has been the target of heavy military shelling and the scene of unquantifiable deaths, including those of Sunday Times journalist Marie Colvin and French photographer Remi Ochlik.
The UN chief said the aim of today's mission was to "urge all sides to allow unhindered access for humanitarian relief workers so they can evacuate the wounded and deliver essential supplies".
Once in Baba Amr she spent around 45 minutes surveying the devastation, accompanied by officials from the Syrian Red Crescent.
Volunteers said most of the inhabitants appeared to have left the stricken district.
Many are understood to have decamped to outlying areas where relief teams have been distributing food parcels, blankets and baby milk.
Despite international appeals, the Syrian government has still refused to allow any aid workers into Baba Amr, claiming security risks.
But activists have accused the government of conducting a "mopping-up" operation to hide its activities.
The International Committee of the Red Cross (ICRC) said: "ICRC can confirm that Baroness Amos accompanied the Syrian Arab Red Crescent (SARC) team into Baba Amr.
"SARC volunteers said most of the inhabitants of Baba Amr appeared to have left. They seem to have departed to other suburbs and outlying areas. Those destinations include areas where ICRC and SARC teams have been distributing assistance in recent days."
The ICRC said it had provided food and hygiene items to 450 families in Abel today, around 10km south of Homs. Many of those displaced from Baba Amr have fled to the area.
"It was cold and it was clear that many of the families we helped had left their homes in a hurry," a spokesman said.
The organisation will provide mattresses and blankets in the coming two or three days.
Regime forces now appear to be turning their attention to other rebellious areas, such as the northern province of Idlib near the border with Turkey.
The shift suggests the Syrian military is unable to launch large operations simultaneously, despite its strong security services.
The British Ambassador to Syria, Simon Collis, said it was clear Assad's regime was "doomed" but a quick resolution was unlikely.
Writing on the UN website, he said: "I think there's been a hollowing out of support for him. People know that, that the regime is doomed.
"Many business people and others have told me that they understand this, they don't want to be part of it. They don't see a future for themselves or their families, but they're fearful of acting right now because the regime is still able to get them or their family.
"So it could go quite quickly because it is brittle, but I think it's more likely it will take some time before this comes to an end, but there's only one way it will end."
More than 7,500 civilians have been killed so far in the crackdown, the UN estimates.