UN Secretary-General Antonio Guterres is alarmed by the resurgence of the M23 rebel group in the Democratic Republic of Congo, his spokesman said Wednesday without directly blaming these fighters for the crash of a UN helicopter that killed eight people.

Six Pakistanis, a Russian and a Serb died when a Puma helicopter with the UN mission MONUSCO crashed Tuesday while on reconnaissance over the troubled eastern region of the country.

Military authorities in North Kivu province have accused the M23 rebel group of downing the chopper, though the faction denies the charge and instead blames the Congolese army. The UN mission has not said whom it blames for the crash.

At UN headquarters in New York, Guterres’s spokesman Stephane Dujarric said a UN probe is under way, with a UN team due to work with Congolese authorities.

MONUSCO will continue to carry out reconnaissance flights in the area and monitor the activities of armed groups, Dujarric said.

Without blaming the M23 group for the crash, Dujarric said at his daily briefing that Guterres “is deeply concerned by the resurgence of M23 activities in the border area between the DRC close to Rwanda and Uganda.”

The helicopter came down in the Tchanzu area of Rutshuru Territory, where the army and the M23 rebels had been fighting the day before.

The M23 — its name derives from “March 23 Movement” — emerged years ago from an ethnic Tutsi Congolese rebellion in North Kivu that was once supported by Rwanda and Uganda.