The deputy UN envoy said the fighters used mortars and grenade-launchers in the May 13 assault in the town of Bangassou.
Deputy UN envoy Diane Corner said the fighters used mortars and grenade-launchers in the May 13 assault in the town of Bangassou, which had until then mostly been spared from violence.
The attack targeted a "MINUSCA base deliberately, in a major way, including using some fairly serious weaponry," Corner told reporters by videolink from Bangui.
"It looks like someone has been financing this."
Corner did not speculate as to who may have bankrolled the offensive, which left at least 115 people dead, including a Moroccan peacekeeper.
Five other peacekeepers -- four Cambodians and a Moroccan -- died on May 8 when their convoy was ambushed not far from Bangassou, in the deadliest attack so far on the mission.
Fighting erupted in two other towns -- Alindao and Bria -- that have left scores dead but those clashes were among factions of the Christian anti-Balaka coalition and of the mainly Muslim ex-Seleka rebels.
In Bangassou, the attack blamed on anti-Balaka rebels targeted the UN base before turning to Muslims in the southeast town, located near the border with the Democratic Republic of Congo.
"This is of a different order of magnitude," said Corner, adding that it points to a "more sophisticated" planning.
Control of resources in the area -- which is rich in diamonds, gold, coffee and cattle trading - may have played a role in fighting, she added.
Hundreds of reinforcements were sent to Bangassou to restore calm, and Corner said the situation remained "extremely fragile."
The United Nations has 10,000 troops and 2,000 people serving in its MINUSCA force in Central African Republic, which descended into bloody sectarian fighting after the 2013 overthrow of leader Francois Bozize.