In this story:

• Rwandan involvement?
• Foreign minister joins rebels
• U.S. worried conflict will spread
• Related stories and sites

KINSHASHA, Congo (CNN) -- Rebels in the Democratic Republic of Congo reportedly captured the eastern region's two largest towns Wednesday as they named a long-time opponent of President Laurent Kabila as their political leader.

The rebel-controlled Voice of the People radio named Arthur Z'Ahidi Ngoma as coordinator of the rebellion. Ngoma is a former member of the U.N. cultural agency UNESCO and was jailed briefly by Kabila last November.

Meanwhile, fighting subsided Wednesday in Bukavu and Goma, both near the Rwandan border. Kabila's government acknowledged losing Bukavu, while rebel radio broadcasts from Goma indicated anti-Kabila forces had taken the town. Rwanda state radio reported that both towns had fallen to the rebels.

The rebellion against Kabila started Sunday in the eastern part of the vast country, formerly known as Zaire, after Kabila asked Rwandan soldiers -- who helped him seize power last year -- to go home.

The Congolese government Wednesday again accused neighboring Rwanda of inciting the rebellion.

"Almost 400 army soldiers have come from Rwanda and have caused Congolese bloodshed," said government spokesman Didier Mumegi.

The newly named rebel leader, however, denied Rwandan involvement and maintained the movement is not ethnically driven.

"This is not a Banyamulenge struggle. It is a struggle of all Congolese," Ngoma said.

The Banyamulenge are ethnic Tutsis of Rwandan origin who helped Kabila topple long-time dictator Mobutu Sese Seko in May 1997 with the support of the Rwandan government.

The rift between the allies developed soon after Kabila took power. The Rwandan government was angry over Kabila's inability to stop attacks on Rwanda by Hutu rebels based in eastern Congo. Kabila retaliated by dismissing many Banyamulenge members from his coalition government.

On Wednesday, Rwandan President Pasteur Bizzimungu denied his country was involved in the conflict, but left open the possibility of future involvement.

"We do not know what the outcome will be like in the Congo ... If there should be any serious reasons to be involved, maybe we should (become involved)," he said.

Despite brief fighting near army barracks on Monday, the capital Kinshasha was relatively calm and quiet. Little has been heard from Kabila this week.

The embattled president suffered another blow Wednesday when it was confirmed that Foreign Minister Bizima Karaha had joined the rebels.

After splitting with Kabila and fleeing to South Africa, Karaha traveled to rebel headquarters in Goma, workers for two aid organizations told CNN.

Karaha, a leader of the Banyamulenge, also denied that the rebellion was backed by Rwanda. Interviewed by telephone from Goma, Karaha accused Kabila of corruption and nepotism and said he had failed to chart a clear political course for Africa's third largest country.

The United States warned Wednesday that the situation in Congo was rapidly deteriorating and urged neighboring countries not to become involved.

"The U.S. is concerned by reports that Rwandan troops are involved in military operations ... in support of a Congolese military rebellion," said State Department deputy spokesman James Foley.

"We urge all countries in the region to respect the territorial integrity of the Congo, refrain from becoming involved in the conflict and respect international law," he said.

African leaders are planning to discuss the Congo crisis at a regional summit in Zimbabwe on Friday. The governments of Congo, Rwanda, Zambia, Namibia and Uganda have expressed a willingness to attend.