The shells hit the outskirts of the eastern city yesterday, killing a woman and a child, and a rocket struck the nearby village of Kanyaruchinya, killing two and injuring nine others, according to the UN.
The civilians were victims of fighting that has erupted sporadically since mid-July between M23 and the regular army in a region 20 kilometres north of Goma, a city of one million people, ending a truce of about two months.
UN military spokesman Prosper Basse told UN-sponsored Radio Okapi that MONUSCO troops were "henceforth engaged alongside the FARDC (regular army)" to deal with the threat.
It marks the first time the UN brigade has fought alongside the army.
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MONUSCO had already declared a protection zone with a perimeter of 30 kilometres around Goma and the nearby strategic town of Sake three months ago "to prevent this sort of atrocity by M23 against civilian populations", spokesman Basse said.
"The DR Congo government is awaiting an explanation from its neighbour over these serious events," said spokesman Lambert Mende.
In a tit for tat exchange, Kigali accused the Congolese army of launching a rocket on Rwanda while the DR Congo said that M23 rebels had done it to implicate the government, and provoke conflict with Rwanda.
M23 meanwhile issued a statement laying the blame on the regular army and an ethnic Hutu rebel movement based in the region, the Democratic Forces for the Liberation of Rwanda (FDLR).