KINSHASA, Congo – U.N. attack helicopters targeted M23 rebels in eastern Congo on Saturday after fighting resumed following a monthslong lull in violence, a local official said.

Two army officers and 151 rebels were killed in a battle beginning Thursday that the U.N. called the worst clash between the M23 group and the military since July. Attack helicopters for the U.N. mission in Congo, known as MONUSCO, had been on standby.

"MONUSCO helicopters this morning bombarded the M23 positions in the city of Kibumba," said North Kivu governor Julien Paluku. He said the Congolese army had earlier retreated from Kibumba, which is 30 kilometers (19 miles) north of Goma, after thousands of Rwandans, who he says were backing the rebels, attacked early Saturday.