A spokesman said there were "worrying" signs of a build-up of Rwandan troops ahead of a possible offensive against a local rebel commander who has split from his former allies.

We've heard there are between 400 and 500 Rwandan troops and rebel soldiers near Moba in northern Katanga

Hamadoun Toure UN spokesman

Meanwhile, a peace deal between the government of Joseph Kabila and another former rebel, Jean-Pierre Bemba is under threat.

Civil society groups have pulled out of peace talks intended to iron out details of the power-sharing agreement signed in April.

The deal was intended to end the four-year-long Congolese war but the biggest rebel group, the Rwandan-backed Rally for Congolese Democracy (RCD), did not sign up.

'Circus'

Forces from Rwandan and their RCD allies are reportedly heading towards the DR Congo's border with Zambia, where fighting has recently been reported near the town of Pweto.

"We've heard there are between 400 and 500 Rwandan troops and rebel soldiers near Moba in northern Katanga and this is very worrying if it marks the beginning of an offensive," UN spokesman Hamadoun Toure said.

Under the agreement signed in April, Mr Bemba was supposed to become prime minister but he has not yet come to Kinshasa to take up his post.

The civil society groups complained that they were being ignored at the Matadi talks.

Kibiswa Naupess, representing the civil society groups, called on the government and delegates from Mr Bemba's Movement for the Liberation of Congo (MLC) to "stop their circus".

Last week, veteran Congolese politician Etienne Tshisekedi toured areas of the country held by the RCD in order to cement the alliance of his Union for Democracy and Social Progress (UDPS) with the RCD.

The war in the Democratic Republic of Congo has drawn in the armies of Angola, Namibia and Zimbabwe on the side of Mr Kabila, while Uganda and Rwanda backed different rebel groups.