Syrian authorities are blocking access to the old city of Homs, where trapped civilians are in dire need of food and medical supplies, the Red Cross said.
The International Committee of the Red Cross (ICRC), in a statement issued on Wednesday, warned of possible "tragic" consequences if aid does not arrive in Homs soon.
Despite lengthy negotiations with both sides, and three trips back and forth between Damascus and Homs, we have still not received the go-ahead from the Syrian authorities.
The agency revealed last Friday that it was negotiating a humanitarian pause to be able to enter Homs, where President Bashar al-Assad's forces have been conducting a heavy offensive against rebels, with air and artillery strikes.
"We have been trying, for close to 20 days now, to bring medical supplies and other aid to the old city of Homs," Magne Barth, head of the ICRC delegation in Syria, said in a statement issued in Geneva.
"Despite lengthy negotiations with both sides, and three trips back and forth between Damascus and Homs, we have still not received the go-ahead from the Syrian authorities," he said.
Homs, in central Syria, is the epicentre of the armed rebellion that grew from popular street protests against more than four decades of Assad family rule.
Some 2,000 people are now believed to be trapped there, aid agencies say.
Reaching tens of thousands of people in areas encircled by government forces or armed opposition groups remains one of the toughest challenges the ICRC faces in Syria, the agency said.
Under international humanitarian law, warring parties are obliged to allow rapid safe passage of humanitarian relief for civilians.
"They must also allow civilians in areas besieged by fighting to leave for safer areas, should they wish to do so.
“Regrettably, these obligations are not always fulfilled," the agency said.
Gesture of goodwill
Meanwhile, in Aleppo, 80 members of the Syrian opposition have been released from Aleppo’s central prison under a deal named “Food for freedom” that was brokered by the Syrian Red Crescent.
Under the agreement, the Ahrar al-Sham movement which has been laying siege to government-controlled parts of the city will ease their blockade to enable the forces get in much needed food supplies.
The release of the prisoners and the deal behind it is seen as a rare gesture of goodwill between the government and its opponents.
"To the Syrian opposition, though, it’s a recognition of their might by the Syrian regime," Al Jazeera's Mohammed Adow said, reporting from Aleppo.
The latest developments came as the head of UN investigation into the use of chemical arms in Syria, Ake Sellstrom, and the UN disarmament chief, Angela Kane, arrived in Damascus on Wednesday to start talks with Assad's government on securing access to sites in Syria where chemical weapons are said to have been used.