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.

“The fighting was very violent between the Congolese military and the M23 rebels backed by the Rwandan army,” he said. “Rwandan forces bombarded our positions in Kibumba since early this morning and an estimated 3,500 crossed the border to attack us.”

Reports by United Nations experts have accused Rwanda and Uganda of supporting the rebels. Both countries strongly deny any involvement and Uganda said if the charges continue it will pull its peacekeeping troops out of Somalia, where they are playing an important role in pushing out the Islamist extremist rebels.

At U.N. headquarters in New York, peacekeeping chief Herve Ladsous said the rebels were very well-equipped, including with night vision equipment, which allowed them to attack at 4 a.m. However, he said that UN officials did not know who was supplying the M23. “At this stage we are not in a position to confirm whether or not Rwanda was directly involved in those attacks.”

Earlier on Saturday, Secretary-General Ban Ki-Moon had called Rwandan President Paul Kagame “to request that he use his influence on the M23 to help calm the situation and restrain M23 from continuing their attack,” Ladsous said.

The U.N. Security Council called for an immediate stop to the violence following a two-hour, closed-door emergency meeting. The council said it would add sanctions against M23 rebels and demanded that rebels immediately stop their advance toward the provincial capital of Goma.

“We must stop the M23” because Goma’s fall “would, inevitably, turn into a humanitarian crisis,” said France’s UN Ambassador, Gerard Araud. He added that U.N. officials would decide in the coming days which M23 leaders to target for additional sanctions.

The U.N. and the United States have both issued sanctions against the M23 leader, Sultani Makenga, who is accused of forcing children into the M23 ranks. The Congolese government said it welcomes sanctions against the M23 but estimates they are not sufficient.

Bosco Ntaganda, who also is thought to be leading the M23 rebellion, is under an International Criminal Court arrest warrant for similar war crimes.

The council also warned that any attempts to “undermine” the U.N. forces’ mandate, which includes protecting civilians, “will not be tolerated.”

The M23 was created after officers from the Congolese army defected in April and May and launched a rebellion to demand better pay, armaments and amnesty from war crimes.

Direct fighting broke out Thursday in Rugari, the town between the M23 and the Congolese army positions, only 30 kilometers (19 miles) from Goma, and around 15 kilometers (9 miles)from Kanyaruchinya, a camp where more than 60,000 people have already sought refuge from the fighting since June. Ladsous, said about 4,000 more people had been displaced by Saturday’s fighting and were seeking refuge at the camp.

The army spokesman in North Kivu, Col. Olivier Hamuli, said Friday that two army officers were killed and seven were wounded in the fighting in the Kibumba area.

M23 spokesman, Col. Vianney Kazarama, denied the heavy casualties, saying that only two rebels were wounded.

The U.N. peacekeeping mission in Congo said it was the worst battle since July.  Paluku said the country faces a humanitarian crisis as those displaced are moving toward Kanyaruchinya.

Since August, members of the International Conference for the Great Lakes Region have been holding talks in Kampala, Uganda, to try to find a solution to the conflict. There had been a de facto cease-fire during the mediation, but tensions mounted on the ground over the past two weeks as the talks seemed to be reaching a dead end.