Soldiers loyal to President Assad have been bombarding Homs, a prominent rebel stronghold, despite Red Cross appeals.
Syrian forces renewed their bombardment Saturday of rebellious areas of Homs, activists said, as the Red Cross vowed to try again to reach thousands of people stranded in a district overrun by regime troops after a monthlong siege.
Conditions in the western neighborhood of Baba Amr have been described as catastrophic, with extended power outages, shortages of food and water, and no medical care for the sick and wounded.
Syrian government forces took control of the neighborhood Thursday after rebels fled the district under constant bombardment that activists said killed hundreds of people since early February.
The Syrian regime has said it was fighting "armed gangs" in Baba Amr, and has vowed to "cleanse" the neighborhood.
The Red Cross said it had received permission from President Bashar Assad's government to enter Baba Amr. A convoy of seven trucks with 15 tons of humanitarian aid including food, medical supplies and blankets left Damascus on Friday, taking several hours in heavy snowfall to reach Homs.
But once they neared Baba Amr, the government prevented them from entering.
"We are still in negotiations to enter Baba Amr," ICRC spokesman Hicham Hassan said Saturday in Geneva.
The Syrians said they were not letting the Red Cross into Baba Amr because of safety concerns, including land mines, Hassan said, adding the organization had not been able to verify the danger. The government has not offered its explanation for revoking the permission.
"It's important that we get in today," Hassan said. "We are not about to give up."
Other areas in Homs, Syria's third-largest city with about 1 million people, came under heavy shelling Saturday, including areas where many of Baba Amr's residents had fled. The Local Coordination Committees activist network said mortars slammed into the districts of Khaldiyeh, Bab Sbaa and Khader.