An aid worker has died from gunshot wounds in eastern Democratic Republic of Congo, where Rwanda-backed rebels who captured two key cities in recent weeks are trying to seize more territory.

Jerry Muhindo Kavali, 49, was injured two days ago by a bullet that struck the Médecins Sans Frontières (MSF) office in Masisi where he was working.

He was taken to hospital in Goma for life-saving treatment but died of his wounds on Saturday.

Colleagues say he was deeply dedicated to humanitarian work and "always had a smile on his face".

Kavali's death has angered aid workers trying to help the tens of thousands of Congolese people whose lives have been destroyed by the war.

"Even war has its rules", said MSF in response to the news.

The town of Masisi, where the MSF worker was shot, has witnessed fierce fighting between militias allied with the Congolese army and rebels belonging to the M23 and Alliance Fleuve Congo rebel groups.

The bullet that killed Kavali was "one of many bullets to hit our premises over recent weeks", the MSF head of programmes Stephan Goetghebuer said.

Key areas already under M23 control include provincial capitals Goma and Bukavu, and the rebels appear to be continuing their advance.

Eastern DR Congo is rich in minerals and this battle for control of the region partly has its roots in the 1994 Rwandan genocide.

It has been riven by conflict for more than 30 years and attempts in recent years to integrate rebels into the Congolese army have failed.

Experts now fear what effects the conflict will have on the wider East Africa region.

On Friday, the UN Security Council adopted a resolution condemning the M23 offensive in eastern DR Congo, urging Rwanda to stop supporting the rebels, and saying they should immediately leave Congolese territory "without preconditions".

This week, the US announced sanctions on M23 Spokesman Lawrence Kanyuka and Rwandan minister James Kabarebe for his alleged role in the conflict.