One of the renegade leaders who captured the eastern Democratic Republic of Congo town of Bukavu has fled to neighbouring Rwanda.
Mutebusi said the UN had tried to kill him
Colonel Jules Mutebusi crossed the border with 305 men, who had been disarmed, a Rwandan spokesman said.
Tensions have risen between DR Congo and Rwanda since Congolese President Joseph Kabila accused Rwanda of backing Col Mutebusi.
The unrest threatens DR Congo's shaky peace, following five years of war.
The BBC's Rob Walker in Rwanda says that despite Col Mutebusi's departure, the crisis in the Congolese peace process continues.
His ally, General Laurent Nukunda, remains at large.
The United States has announced that it is sending an envoy to the two countries and has appealed for restraint.
"We are deeply concerned about the build-up of forces in eastern Congo," said US State Department spokeswoman Brenda Greenberg, announcing the diplomatic mission by Deputy Assistant Secretary of State Donald Yamamoto.
South African President Thabo Mbeki described the possibility of war between DR Congo and Rwanda as "potentially catastrophic".
On Monday, the Congolese army said it had forced Col Mutebusi's men out of the border town of Kamanyola, about 40km south of Bukavu.
UN peacekeepers were also involved in the fighting, firing from helicopter gunships.
"You wanted to kill me, that's why I fled" and surrendered to the Rwandan army, Col Mutebusi told two UN military observers who interviewed him, AP news agency reports.
Rwanda had earlier said that it had closed its border with DR Congo to prove that it was not backing the rebels.
But army spokesman Colonel Patrick Karegeya said Rwanda had let Col Mutebusi and his men across because they were fleeing fighting.
"We are just abiding to international humanitarian conventions," he said.
Rwanda said it was worried by the "offensive" positions being taken by thousands of extra Congolese troops sent to the east, following the rebel capture of Bukavu.
During the war, Rwanda backed the main rebel group, the RCD, to which Col Mutebusi belongs, along with Gen Nkunda.
Under a peace deal signed last year, the former rebel groups are supposed to be integrated into a new national army but progress has been slow.
An estimated three million people in Congo were killed during more than five years of war which sucked in neighbouring countries.
Some 36,000 people have fled the recent unrest
And tensions between the former enemies were one factor in the Bukavu fighting.
A Congolese military spokesman did not give details of the deployment in the east but said that the army was "doing its job" following the recent unrest.
"After the problems we have had, the president called for a general mobilisation. This is what we are doing," Colonel Leon Kasonga said.
Gen Nkunda had said that his fellow ethnic Banyamulenges were being targeted and killed by the army, but the UN dismissed his claims that he was preventing a genocide.
The Banyamulenge are ethnic Tutsis, who have lived in DR Congo for several generations but who retain ties to Rwanda.
Since the Congolese army retook Bukavu, some 36,000 Banyamulenge have fled to neighbouring Burundi.