At least 59 people are now reported to have been killed in the continuing government bombardment of the Syrian city of Homs on Sunday.
The centre has been under siege for more than three weeks.
The International Committee of the Red Cross has been trying to negotiate a halt to the fighting in order to evacuate the wounded.
Talks with the Syrian authorities and the opposition are still ongoing.
The latest bloodshed coincided with a referendum on a change to the constitution.
According to state television, 89 percent of Syrians backed the new proposals but the entire process has been dismissed as a sham by opposition groups.
The new constitution proposes holding multi-party elections within three months and limiting presidents to two seven-year terms.
It could leave Assad in power until 2028.
Syrians vote on new constitution amid bloodshed