The BBC's Chris Simpson in Kigali: Little prospect of a ceasefire

Clashes between the two sides - which are supporting rival wings of the rebel Congolese Rally for Democracy (RCD) - erupted in the strategic north-eastern city on Saturday.

An informal ceasefire called on Monday afternoon held for just a few hours. Residents were reported to have suffered another night of heavy artillery and mortar fire.

Commanders on both sides accused each other of breaking the ceasefire.

Ugandan Colonel Otafiire Kahinda said: "There was intense fighting overnight and it is still going on.

The RCD is trying to oust Laurent Kabila

"We talked and discussed but the Rwandans don't accept the ceasefire. We are still in our positions and they are attacking us."

But a senior Rwandan commander blamed the Ugandans for breaking the peace. He said Ugandan troops escorted by tanks tried to shoot their way into town on Monday evening from a base at the city's international airport, but were forced back.

"The force was intercepted. We blocked the road with stones and obstacles and scattered them and now we are hunting them down. That's why right now there is some shooting," he said.

Both sides said they had the upper hand in the fighting, but the reports could not be independently verified.

(Click here to see a map of the region)

The city's streets are said to be deserted, with remaining civilians hiding in their houses.

Unconfirmed reports on Monday estimated that some 50 people had been killed in the fighting since it broke out on Saturday at Bangoka international airport.

Ugandan President Yoweri Museveni and Rwandan Vice-President Paul Kagame met for a second round of talks at a resort in western Uganda on Tuesday in an attempt to resolve the dispute.

Withdrawal urged

For its part, the Congolese Government has urged the United Nations Security Council to hold an emergency meeting to call for an immediate halt to fighting between the Ugandan and Rwandan forces and their withdrawal from its territory.

In a letter to the Security Council president, Congolese ambassador Andre Kapanga said the Council must put pressure on Kampala and Kigali to get the rebels to sign the Lusaka ceasefire agreement.

Rivalry between the two factions blocked the rebels from signing a peace accord brokered on 10 July in the Zambian capital which was signed by six African governments involved in the year-old conflict.

Mr Kapanga also urged the Council to "state and condemn" the interruption of a polio vaccination programme set to protect 10 million children in the country.

Fighting in DR Congo has trapped women and children in clinics where they came for the vaccination programme.

UN Secretary-General Kofi Annan said on Monday that the fighting violated a truce which he had requested and which all parties had endorsed to allow the vaccination programme to continue unhindered in August.

The conflict in DR Congo is expected to dominate talks between southern African leaders at the Southern African Development Council summit in Mozambique.