Rwanda-backed M23 rebels in Democratic Republic of Congo executed at least 21 civilians over two days in February in the eastern city of Goma, Human Rights Watch said in a report published on Tuesday.

The report covers incidents on February 22-23 in a Goma neighbourhood, offering a snapshot of the violence during the latest escalation of the decades-long conflict.

“Commanders and combatants who directly ordered or carried out abuses should be held criminally accountable,” Human Rights Watch said in the report.

M23 rebels have seized eastern Congo’s two largest cities, Goma and Bukavu, in an offensive that began in January. The unprecedented advance has killed thousands of people and forced hundreds of thousands to flee.

The 21 slain civilians included six men and one woman shot in the head near Katindo military camp in Goma on February 22. HRW said M23 were responsible, citing a witness.

In a separate incident, M23 killed people and dumped their bodies at a construction site less than 100 metres away from the camp. These included a 15-year-old who was taken from his home and later found dead at the site, HRW said, citing a relative and a neighbour.

Goma's Kasika neighbourhood was targeted because it had previously housed Congolese army barracks, HRW said.

The violence continued in the neighbourhood a day later on February 23 when M23 rounded up around 20 young men at a nearby sportsfield.

A witness told HRW the rebels accused the young men of being members of the army. Three that tried to run away were shot.

An M23 leader said the group would look into the allegations and publish the findings.

“HRW gives us an opportunity to mirror ourselves. We respect this organisation despite its past accusations which proved to be false,” M23 leader Bertrand Bisimwa told Reuters.

The overall toll in Goma is likely to be higher, HRW said, with medical workers reporting that over 50 bodies were collected from the Kasika area over the two-day period.

Other organisations have previously reported grievous crimes committed in eastern Congo since M23 seized swaths of territory.

Amnesty International in March said rebels had raided hospitals in Goma for wounded Congolese soldiers and taken 130 people including caregivers. Many were tortured and some are still missing.

UNICEF reported a five-fold surge in rape cases treated across 42 health centers in eastern Congo, in February, describing it as the worst sexual violence seen there in years. Almost a third of the victims were children, UNICEF said.

In one example, a mother reported that her six daughters, the youngest just 12 years old, were raped by armed men searching for food.

The conflict in eastern Congo is rooted in the spillover of Rwanda's 1994 genocide and the struggle over Congo's vast mineral resources. Rwanda denies UN allegations that it backs the M23, saying its forces are acting in self-defence against Congo's army and allied militias.