A Romanian "mercenary" and two Congolese soldiers were killed and a UN peacekeeper wounded in three separate incidents in eastern Democratic Republic of Congo, several sources said on Sunday.

A security official in the east of the country told AFP on condition of anonymity that a private military contractor was killed and three others wounded on Saturday by a missile strike on a Congolese army base around 10 kilometres (6 miles) north of Goma.

The origin of the strike was not confirmed.

The capital of North Kivu has been surrounded to the north and west by the Rwandan army and M23 rebels for several months.

Fighting regularly takes place against the Congolese army on the outskirts of the city, while the rebels, backed by Kigali, continue to extend their hold in the east of the country.

The Romanian Ministry of Foreign Affairs said on Sunday that the deceased and two injured people were Romanian nationals and that the fourth injured person was of another nationality.

Several Romanian television channels -- including the state-owned TVR Info -- described the dead fighter as a "Romanian mercenary" under contract to the Congolese army.

Around 200 kilometres north of Goma, in Butembo, at least two soldiers were killed in an ambush, local administrator Colonel Alain Kiwewa told AFP.

He said he did not yet have any details of the identity of the assailants and that an investigation had been opened.

But Kiwewa said the killed soldiers had been supplying others fighting the ADF rebels.

Since the start of the moth, around 150 people have been killed in attacks attributed to the ADF, which originated in neighbouring Uganda and most of whose members swear allegiance to the Islamic State Group.

Also around Butembo, a MONUSCO (UN mission in the DRC) convoy was attacked by unidentified armed men on Saturday evening as it returned from a mission.

One peacekeeper was shot in the leg during the attack.

Vivian van de Perre, second-in-command of the UN mission, condemned "the violence perpetrated against peacekeepers" and reiterated the need for "unhindered access for the protection of civilians".

On Sunday, Pope Francis deplored the surge in violence in eastern DRC and appealed to the national authorities and international communities to "safeguard the lives of civilians".