Burundi pulls its troops from eastern Congo as M23 rebels gain ground, seizing major cities. The UN reports child executions by rebels. Thousands flee to Burundi, facing overcrowded shelters. Uganda steps in to stop militia killings.

Burundi is withdrawing its forces from eastern Congo where they had been fighting against M23 rebels, four sources said on Tuesday, in a further blow to the Democratic Republic of Congo army as it struggles to halt a rebel advance.

The pull-out came as the U.N. human rights office accused M23 rebels of executing children in eastern Congo during their advance, which has seen the group seize the region's two largest cities.

"The Burundian troops are withdrawing from Democratic Republic of Congo. A number of trucks filled with military arrived in the country since yesterday" through a border post, a Burundian army officer said, confirming movements also described by two U.N. sources and an African diplomat.

Burundian soldiers fought alongside the Congolese to try to defend Kavumu, home to the airport that services Bukavu, the capital of South Kivu province which fell over the weekend. It was the rebels' most significant prize since they seized Goma, the biggest city in east Congo, in late January.

Burundi has had soldiers in eastern Congo for years, initially to hunt down Burundian rebels there but, more recently, to aid in the fight against M23.

An M23 source said on Tuesday afternoon that not all Burundian troops were gone, and a resident of South Kivu said some were still on the Congolese side of the border.

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.

Various militias are vying for control of the region's abundant mineral resources such as tantalum and cobalt, key components of electric vehicles, mobile phones and other technological products used across the world.

Congo is also the top supplier of copper to China.

EXECUTIONS

In Geneva, the U.N. Rights Office said conditions were deteriorating sharply for civilians caught up in the fighting in eastern Congo, with reported abuses including summary executions and sexual violence.

"Our office has confirmed cases of summary execution of children by M23 after they entered the city of Bukavu last week," spokesperson Ravina Shamdasani told a press briefing.

Three boys likely no older than 15 were killed during an altercation with rebels after they refused to give up weapons they had taken from an abandoned military camp, she said.

Between 10,000-15,000 people have crossed into Burundi from around Bukavu in recent days, straining resources and leading to overcrowding in transit centres, Matthew Saltmarsh, spokesperson for the U.N. refugee agency, said.

Some have drowned in the Ruzizi River while trying to cross, he said.

"(Refugees) are exhausted and traumatized. Many of them have been separated from their families with little information on their whereabouts," Saltmarsh said.

Meanwhile boat traffic returned to Lake Kivu on Tuesday as ports re-opened in Goma and Bukavu, which the United Nations said could facilitate access to humanitarian aid after weeks of fighting and looting.

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.

North of Goma, Uganda's army said it had entered the eastern Congolese city of Bunia with approval from local military authorities to stop militia killings there.

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