The M23 rebel group in the Democratic Republic of Congo's volatile east has summarily executed 29 civilians since mid-June in areas under their control, Human Rights Watch said Monday.

The M23 -- or "March 23 Movement" -- leapt to prominence when it briefly captured the eastern Congolese city of Goma in 2012 before being driven out in a joint UN-Congolese offensive.

After lying mostly dormant for years, the group resumed fighting last November. The rebels have since made significant advances in eastern Congo. 

Last month, M23 fighters captured the strategic town of Bunagana on the Congolese-Ugandan border.

"Witnesses told Human Rights Watch that on June 21, following fighting around the village of Ruvumu, M23 rebels summarily killed at least 17 civilians, including two teenagers, whom they accused of informing the Congolese army about their positions and hideouts," HRW said.

"Some were shot dead as they attempted to flee, while others were executed at close range," the rights monitor said.

The other deaths took place in subsequent attacks on the same village and in the hamlets of Ruseke and Kabindi, taking the death toll to 29, HRW said.

"Since the M23 took control of several towns and villages in North Kivu in June, they've committed the same kind of horrific abuses against civilians that we've documented in the past," said HRW's senior Congo researcher Thomas Fessy.

The DRC has repeatedly accused neighbouring Rwanda of backing the M23, a charge the small central African country has always denied.

HRW said there were "heightened concerns" that M23 "is receiving Rwandan support for its operations in North Kivu province".

"Donor countries should suspend military assistance to governments found to be supporting the M23 and other abusive armed groups," the New York-based body said.

HRW urged the UN, the African Union and DR Congo's international donors to "support a clear strategy to address impunity for serious abuses."

Such a strategy should include "a vetting mechanism for the security and intelligence services, an internationalised justice mechanism, and a comprehensive reparations programme, as well as an effective demobilisation programme," it added.