Despite a 2003 peace deal, the east has remained unstable
Evidence is increasing that foreign forces are being drawn into the conflict in the east of the Democratic Republic of Congo.
Eyewitnesses told the BBC Angolan and Zimbabwean troops were on the ground.
Meanwhile, journalists report that some of Laurent Nkunda's rebel fighters are in the pay of the Rwandan army.
This has renewed fears that the fighting will see a re-run of the five-year Congolese war, which involved nine nations, before it ended in 2003.
Some 250,000 people have fled the most recent violence which flared in August between government forces and rebels demanding protection from Rwandan Hutu rebels who fled to DR Congo after Rwanda's 1994 genocide.
Gen Nkunda's rebels have told AFP news agency that they have advanced to the outskirts of Kanyabayonga, 100km (60 miles) north of the regional capital, Goma, where government forces were accused of looting and raping civilians earlier in the week.
The UN has accused both sides of war crimes during the latest upsurge in violence.
Despite a peace deal, the presence of the world's largest peace force and UN-backed elections, the resource-rich east of the country remains unstable.
According to a resident of Goma, who asked to remain anonymous for his own safety, there are soldiers supporting the Congolese government in the town who are Angolans
"We are seeing soldiers wearing Congolese army uniform here in town but they are not speaking the same language like us. They are unable to speak French, Swahili or Lingala - that is bizarre," he told the BBC's Network Africa programme.
"They are patrolling but unable to communicate with the population. These are speaking Portuguese," he said.
FORCES AROUND GOMA
CNDP: Gen Nkunda's Tutsi rebels - 6,000 fighters
FDLR: Rwandan Hutus - 6,000-7,000
Mai Mai: pro-government militia - 3,500
Monuc: UN peacekeepers - 6,000 in North Kivu, including about 1,000 in Goma (17,000 nationwide)
DRC army - 90,000 (nationwide)
Source: UN, military experts
Experts say this evidence is not conclusive, since some Congolese troops, including a group known as the Katangan Tigers, fought in Angola during their civil war, and frequently converse in Portuguese.
Angola denies sending troops into DR Congo, although says it would support the government if called on to do so by the regional grouping, the Southern African Development Community.
During the last war, Congolese government forces supported by troops from Angola, Zimbabwe and Namibia, were fighting rebels backed by Uganda, Rwanda and Burundi.
Forces from Chad and Central African Republic were also involved in that conflict.
The war, which erupted in August 1998, was the largest on the continent in recent times.
A recent Zimbabwean army deserter told the BBC he had been part of a force that remained in DR Congo after the end of the last war in 2003.
UN peacekeepers have been accused of not doing enough
"There are about 250 soldiers who were left behind without knowledge of other countries," he said.
"Those who were left there are there to do duties like guarding President Kabila and those mines where they have got some interests of our bosses."
He said he had been on duty in Goma and the mining centre Lubumbashi and soldiers were rotated about ever six months.
And a few months ago, Gen Nkunda's fighters said they captured two Zimbabwean soldiers along with some Hutu FDLR rebels fighting with the Congolese army.
Meanwhile, Britain's Financial Times newspaper says soldiers from the Rwandan army have been fighting alongside General Nkunda.
Their reporter in eastern DR Congo interviewed former rebels and observers who said some soldiers were continuing to receive their Rwandan salaries while fighting with the general.
Earlier this week, Joseph Mutaboba, an aide to the Rwandan president, told Reuters news agency there was a "possibility" that demobilised Rwandan troops were fighting in DR Congo
Rwanda has repeatedly denied sending forces into DR Congo.
BBC Africa analyst Martin Plaut says similar statements were made during the early stages of the last war - and no-one doubts that foreign troops played a key role in that conflict.
But former British Prime Minister Tony Blair, an adviser on governance to the Rwandan leader, says the country has "absolutely no interest in either stoking this or making a cause of instability on its border".
"Neither country in truth has any interest at all in having this humanitarian disaster unfold and these forces continue," Mr Blair told the BBC's Focus on Africa programme on a visit to Rwanda.
He called for disbandment of all armed groups in the east, as agreed in last year's Nairobi agreement, and said Gen Nkunda's promise to march on the capital was "plainly absurd".