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.
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.