A spokesman for the Congo government, Henri Mova Sakanyi, said his nation wanted to resolve the situation diplomatically but would be "obliged to react" if foreign troops crossed its border.
Burundi's army chief also accused Congolese soldiers of participating in Friday's massacre, which witnesses said was launched from Congo.
The United Nations said it suspended talks with the Burundian Hutu rebels who claimed responsibility for the killings because "it seems they are not willing to contribute to the peace process."
U.N. peacekeepers were helping local forces step up security along each side of the Burundi-Congo border and U.N. helicopters are patrolling the area, U.N. associate spokesman Stephane Dujarric said at U.N. headquarters in New York.
Dujarric called on Burundian authorities to "exercise restraint."
Burundi army Brig. Gen. Germain Niyoyankana said troops were prepared to cross into Congo to pursue the rebels.
"We must avoid a new attack from Congo so the Burundi army does not rule out an offensive in ... Congo. Everything depends on the Congolese government," Niyoyankana said. "Our president has asked the Congolese government for an explanation."
Civil wars between majority Hutus and minority Tutsis in Burundi, Rwanda and Congo have ravaged the region for more than a decade. The conflicts include the 1994 genocide in Rwanda in which 500,000 people - mostly Tutsis - were killed, an ongoing civil war in Burundi and a five-year war in Congo.
Rwanda and Burundi twice have invaded Congo trying to root out Hutu militias. The second invasion, in 1998, sparked a war in Congo involving six African countries. An estimated 3.5 million people died, most from war-induced disease and starvation, during the conflict.
Congo Defense Minister Jean-Pierre Ondekane said Rwanda and Burundi "are free to make any declarations they want. They are sovereign states."
But he said "invading our country isn't a solution. They (Rwanda and Burundi) were in Congo but they did nothing to disarm the rebels."
Rwandan Foreign Minister Charles Muligande said his country was ready to act against "a coalition of Burundian, Rwandan and Congolese extremist groups" based in Congo that is bent on eliminating minority Tutsis.
"The status quo cannot be maintained," Muligande, a Tutsi, said in the Rwandan capital, Kigali. "We will not wait to be exterminated."
Niyoyankana also said Congo's army was involved in the massacre, in which refugees were shot, hacked, stabbed and burned to death.
He said Burundian rebels guided the attackers, who included former Rwandan soldiers and Congolese tribal fighters known as Mayi Mayi, who are part of Congo's army.
Mayi Mayi official Muzuri Tambwe denied that accusation, saying in the Congolese capital, Kinshasa, that "it wasn't possible that they could have entered Burundian territory."
The rebel National Liberation Forces said its fighters staged the attack because Burundian soldiers and Congolese Tutsi militiamen were hiding at the camp.
A renegade Congolese army commander, whose forces briefly seized a key eastern Congo city in June, threatened to oust the "government that slaughters its own people."
"In coming days, I will launch a process that will put my words into action," Brig. Gen. Laurent Nkunda, a Tutsi, said in a statement issued from remote eastern Congo.
"That is to say, act in a way, that by any means, cleans the slate, and put in place a true government that is inclusive, consensual, non-conflictual with the aim of installing in Congo the peace of the brave and not the silence of the cemeteries."
The United States condemned the massacre and called for a prompt U.N. investigation. The State Department also said authorities in Burundi and the Democratic Republic of Congo should bring the perpetrators to justice.
The U.N. had been brokering peace talks between Burundi's government and the National Liberation Forces - the last rebel group still fighting that country's 11-year civil war, said
Isabelle Abric, spokeswoman for the U.N. mission in Burundi.
"The negotiations have been suspended because they are claiming responsibility for the attack," Abric told The Associated Press.
U.N. troops are being sent to increase security around the four camps for Congolese refugees, Abric said.
By Aloys Niyoyita