The Rwanda-backed M23 rebel group is trying to show it can administer the cities of Goma and Bukavu as government and U.N. officials warn of a looming humanitarian catastrophe including potential outbreaks of cholera and other diseases.

The World Food Programme said looters stole some 7,000 metric tons of food supplies as the Congolese military withdrew from Bukavu and rebels entered the city over the weekend.

A passenger boat from Bukavu to Goma left on Tuesday morning, operator Lweni Ndale said, adding it was the first he had sent since late January, shortly after Goma fell to M23.

The U.N. humanitarian office said the resumption of boat traffic would allow aid groups to get supplies to Idjwi island on Lake Kivu where more than 100,000 people had sought refuge from fighting.

However the airport in Goma, which the U.N. describes as a lifeline for aid, remains closed, which could make it difficult to scale up operations.

The arrival of M23 in Bukavu has dealt another blow to Kinshasa's authority and escalated a conflict that has fanned fears of an all-out regional war.

Congolese and allied Burundian troops withdrew from the city to avoid fighting in densely populated areas.

The well-equipped M23 is the latest in a long line of ethnic Tutsi-led rebel movements to emerge in Congo's volatile east.

Rwanda rejects allegations from Congo, the United Nations and Western powers that it supports the group with arms and troops. It says it is defending itself against the threat from a Hutu militia, which it says is fighting with the Congolese military.

Congo rejects Rwanda's complaints and says Rwanda has used its proxy militias to loot its minerals.

Congo's ambassador to the U.N., Zenon Mukongo Gay, blamed "inaction" by the U.N. Security Council for the fall of Bukavu and the reinforcement of rebel troops in the area, in a letter to the council's president seen by Reuters on Monday.

Reporting by Congo Newsroom; Additional reporting by Sonia Rolley in Paris and Michelle Nichols in New York; Writing by Robbie Corey-Boulet; Editing by Angus MacSwan

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