A seven-truck Red Crescent convoy, accompanied by the UN aid chief, was allowed to enter the Baba Amr district in the Syrian city of Homs on Wednesday.
UN aid chief Valerie Amos said the aim of her visit was to "to urge all sides to allow unhindered access for humanitarian relief workers so they can evacuate the wounded and deliver essential supplies."
Amos' request to visit Homs early last week was rejected by Syrian officials, whose forces were then in their fourth week of pounding the city with tank and sniper fire.
The Syrian military finished driving armed rebels from the battered Baba Amr district last week, but refused to allow aid workers to deliver relief supplies and remove the wounded citing "security concerns."
Human rights activists in the city reported that Syrian forces were conducting "mop-up operations” that included the rape and execution of residents.
Wednesday's visit by Amos and International Committee of the Red Cross (ICRC) workers marks the first time independent witnesses have been allowed into Baba Amr since the rebels withdrew.
The visit came as the regime of Syrian president Bashar al-Assad continued to shell other towns it says are harboring rebels from the Syrian Free Army (SFA).
At least six people were killed as Syrian forces on Tuesday launched a major assault on Herak, a town in the southern province of Daraa.
A young girl was shot dead by a sniper, while at least five soldiers were killed in clashes with members of the rebel Syrian Free Army, the Syrian Observatory for Human Rights said.
"Large military forces, including tanks and armored troop carriers, launched an assault on Herak. Explosions and heavy machine gun fire were heard," the Britain-based monitoring group said in a statement.
A video posted online on Wednesday showed what activists say is the inside of the Abu Bakr Al Saddiq mosque in Herak. The video showed rubble littering the mosque's entrance, doors blown from their frames and shattered glass covering the floor. An unidentified man says a tank fired on the mosque after residents sought refuge there.
Across the country, 21 people were killed on Tuesday, including 13 from two families in the Baba Amr area of the central city of Homs, the Local Coordination Committees of Syria said on its website.
Reports on Wednesday said clashes in Herak were ongoing, as was the now three-day shelling of the city of Rastan where a family of seven was killed by a tank shell earlier this week. Over the weekend, Assad's forces summarily executed 47 army defectors in Rastan.
Meanwhile, Assad vowed to press ahead with his campaign to crush "terrorism".
"The Syrian people ... have again proven their capacity to defend the nation and to build a new Syria through their determination to pursue reforms along with the fight against foreign-backed terrorism," he said, quoted by state news agency Sana.
European governments are discussing whether to expel Syrian ambassadors as human rights groups say the civilian death toll in Assad's year-long crackdown has reached 8,458.
France's ambassador to Damascus returned to Paris on Teusday after closing down the embassy in Syria.