The government of President Laurent Kabila has accused Rwanda and Uganda of invading his country and Angola, Zimbabwe and Namibia have admitted sending their troops to Congo to help fight the rebels.
But what strategic interests do outside nations have in Congo, and what positions have some of the other neighbouring countries taken?
Rwanda helped President Kabila overthrow the former ruler, Mobutu Sese Seko, last year. But the alliance did not last long.
Rwanda accused Kabila of supporting the Hutu extremists, Interahamwe, who were responsible for the massacre of tens of thousands of Rwandans in 1994, and of discriminating against the ethnic Tutsis that had helped him to power.
Uganda says it has troops in DR Congo but says they are there to fight Ugandan rebels - the Allied Democratic Forces - who have rear bases on Congolese territory.
President Museveni has warned he may intervene in the conflict to protect Uganda's interests if other countries side with President Kabila.
Angola has one of the best trained and equipped armies in the region.
It shares a huge border with Congo, and is concerned that supplies for the former rebel movement, Unita, are passing through Congo.
|Angola is concerned about Unita supplies|
Angolan troops are reported to be in Congo advancing on the rebels from the Atlantic Ocean near Kitona.
Zimbabwe's enthusiastic response to President Kabila's appeal for help came as a surprise to most Africa analysts.
The Zimbabwean military has sent troops to Kinshasa to support President Kabila.
It announced that its air force helped destroy a five-kilometre column of rebel tanks and armoured cars in the south-west of the country.
Zimbabwe appears to have no immediate strategic concerns: it does not share a border with Congo.
Within Zimbabwe there has been opposition to military intervention because of fears that the economy could not support it.
Zimbabwean officials are describing their intervention as an attempt to restore law and order - but the BBC Africa Service's Tom Porteous says that support is more likely to be based on personal relationships between the leaders.
President Kabila also owes Zimbabwe a lot of money for military equipment supplied on a commercial basis.
Kenya is generally pro-Kabila, being suspicious of the Tutsi-led rebellions in the region. When President Mobutu fell, there were demonstrations in the streets of Nairobi, demanding the resignation of President Moi.
- South Africa
The South African President, Nelson Mandela, has criticised the involvement of Zimbabwean troops in Congo and and has called for a negotiated cease-fire.
Mr Mandela has said his country will not aggravate the situation by sending a military force.
South Africa has a huge economic interest in Congo's diamond and mining industries, and wants to create stability in the country.
Zambia shares a long border with the richest part of Congo.
It supported President Kabila when he came to power - but discreetly - and its allegiances remain ambiguous.
The Namibian presdient has said Namibian troops have been sent to Congo to support Mr Kabila.
Namibia's motivations for supporting Congo are also a mystery to most experts.
The BBC's Tom Porteous says that the reasons for many alliances may be based on the commercial interests of large multi-national companies and private security firms - rather than political interests.