"The bombardment of Khaldiyeh intensified this morning with an average of three shells a minute," the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.
The rebel district of Bayada was also shelled, Abdel Rahman said, adding that it was the fiercest shelling of Homs since a UN-backed ceasefire went into force at dawn on Thursday.
The security forces now control some 70 percent of the flashpoint city, which has seen some of the biggest losses of life of the 13-month uprising in Syria.
Rebel fighters remain entrenched in several mainly Sunni Muslim neighbourhoods of the Old City, Abdel Rahman said.
Three civilians died in shelling of Homs on Saturday, among 14 people killed nationwide ahead of a UN Security Council vote approving the dispatch of the observer mission to monitor the shaky truce.