"Several houses were torched and heavy gunfire was heard in the Christian districts besieged by armed Muslims," he said, adding that several hundred people fled the attacks in Bangui. "Men, women and children were running in all directions," he said.
Christian militiamen and army soldiers "took positions... to protect the residents," the source said.
A diplomat said some of the UN and French forces who intervened were engaged sporadically by small groups.
"There is already a consensus that these operations be held in December," she said, adding that while the latest upsurge in violence "perhaps does not jeopardise" the elections, it was "worrying".
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She said the government was "counting on" UN peacekeepers and French soldiers, as well as the army, to "halt the current violence and especially to protect the people who feel a bit abandoned."
The National Authority for Elections' latest timetable proposes a constitutional referendum on December 6, to be followed by a first round of presidential and legislative elections on December 13. If a second-round run-off is called for, it would take place on January 24, 2016.
Samba-Panza said "the legal timeframe and the current situation" precluded completing the elections this year, adding: "We have to be realistic."
The fresh unrest came two days after four people were killed and around 20 wounded in Bangui in a reprisal attack avenging the deaths of two Muslims.
One of the poorest and most unstable countries in Africa, the landlocked former French colony plunged into chaos after president Francois Bozize was ousted in a coup in March 2013.