Conflict in Congo has sent 63 000 refugees fleeing to neighbouring Burundi in its largest such influx in decades, with conditions dire at a crammed stadium camp and many stuck in fields outside, the UN said on Friday.

About 45 000 displaced people are sheltering in a crowded open-air stadium in Rugombo, a few km from the border with Democratic Republic of Congo where the Congolese army and M23 rebel group are fighting.

"The situation is absolutely dire. Conditions are extremely harsh," Faith Kasina, the regional spokesperson for East and Horn of Africa and Great Lakes, told reporters in Geneva.

"The stadium is literally bursting at its seams and there is no additional space for shelter."

Sanitary conditions inside the stadium are said to be poor with only 10 to 15 stalls of latrines for tens of thousands of people. Many families are being forced to camp in open fields nearby, according to the agency.

"Numbers keep swelling, it's a race against time to try and save lives," said Kasina, adding that the needs are fast outpacing the aid being provided.

The refugees include a large number of unaccompanied children separated from their families, the agency says.

On 21 February, UNHCR told a press briefing in Geneva that it would seek to move people from the stadium. However logistical challenges mean it takes six to eight hours to move large numbers of people to the Musenyi refugee site in southern Burundi. That site, which can host 10 000 people, is now 60 per cent full, according to the agency.

The agency has urged countries to contribute to its emergency appeal for $40.4-million for lifesaving help to support the potential influx of 258 000 refugees into Burundi, Tanzania and Zambia.

The M23 advance is the gravest escalation in more than a decade of the long-running conflict in eastern Congo, rooted in the spillover of Rwanda's 1994 genocide into Congo and the struggle for control of Congo's vast mineral resources.

Rwanda rejects allegations by Congo, the United Nations and Western powers that it supports M23 with arms and troops. It says it is defending itself against the threat from a Hutu militia, which it says is fighting with the Congolese military.

Burundi has had its own soldiers in eastern Congo for years, initially to hunt down Burundian rebels there, but more recently, to aid in the fight against M23.