Troops from various African nations arrived in Kinshasa on Friday to help the Kabila government

GOMA, Congo (CNN) -- With rebel troops nearing the capital of Kinshasa, a rebel leader said Angola's entry into the war in Congo has made chance of a negotiated settlement "almost impossible" when a peace summit resumes Sunday.

After massing on the border, Angolan troops entered the war Saturday on the government's side, attacking the rebel troops from the rear, according to Bizima Karaha, Congo's former foreign minister who defected to the rebels.

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"Angolan troops have invaded Congo," he said.

Karaha, speaking from the rebels' eastern stronghold of Goma on Saturday, said Angolan involvement would complicate the crisis, and rebels would strike back.

"We are calling upon the people of Congo to hold the aggression," Karaha said. "Angolans must come to their senses and remove forces from Congo immediately."

Angola's entry into the civil war has made chances of a negotiated settlement "almost impossible," he said.

Congolese Information Minister Didier Mumengi denied that Angolan troops had arrived, and said that when they do, it would be announced.

"When Angola sends its troops, that will be a big day, as was the case with Zimbabwe," Mumengi said on state-controlled television.

A boy carries a toy weapon on his brother's shoulders during a pro-Kabila march in Kinshasa

Power to Kinshasa was cut Saturday by the advancing rebels, who have controlled the giant Inga hydroelectric dam since Monday.

Other countries are involved or threatening involvement. Uganda's President Yoweri Museveni said his country's troops would support the Tutsi-led rebels if other countries did not withdraw.

President Robert Mugabe of Zimbabwe has committed up to 600 troops and four MIG jet fighters to support Congo President Laurent Kabila's troops.

Rwanda also has been accused by the Congo of invading in support of the rebels but has denied involvement.

Karaha said the rebels downed two Zimbabwean planes over rebel territory in Congo's southeast. The report could not be independently confirmed, and Zimbabwe denied its planes were in the Congo.

In the Zimbabwean capital, Harare, Mugabe's spokesman rejected Congolese rebel claims.

George Charamba said Zimbabwe had not deployed its Chinese-made MiG-21 interceptors and ground troops in support of Kabila, the official Zimbabwe news agency reported.

"We have no MiG fighters in DRC (the Democratic Republic of Congo) ... our deployment in the DRC did not have that dimension," the agency quoted Charamba as saying.

British Royal Marines practice landing drills in preparation for possible evacuation of foreign nationals from Congo

Kabila and Mugabe boycotted a peace summit called by South African President Nelson Mandela in Pretoria, South Africa. Kabila cited ill health but did send two Congo officials to the talks, which were scheduled to resume Sunday.

"We are moving ever closer to a full-scale regional war -- that is the fear everyone has," said a source close to the talks quoted by Reuters. "Everything has to be done to avoid that situation."

Earlier Saturday, Mandela met with Rwandan President Pasteur Bizimungu and Museveni. Kabila has accused both of supporting the rebels.

Kabila came to power by toppling former dictator Mobutu Sese Seko in May 1997 with the help of Tutsi troops. Rwanda, Uganda and Angola all backed Kabila in the war against Mobutu. Congo was known as Zaire under Mobutu's rule.

But the rebels, composed of minority ethnic Tutsis, disenchanted soldiers and soldiers from the military of ex-dictator Mobutu, say Kabila turned against them, and accuse Kabila of power-grabbing, tribalism and mismanagement.

The rebels launched their revolt on August 2 in the east shortly after Kabila ordered all Rwandan soldiers to leave the country.

The Associated Press and Reuters contributed to this report.