Some 66,000 Congolese refugees have poured across the border into Uganda after a surprise attack by an Islamist rebel group, aid agencies say, stretching humanitarian resources to breaking point.

The Uganda Red Cross Society said the influx began after the Allied Democratic Forces launched a deadly assault on Kamangu in the eastern Democratic Republic of the Congo last week.

The refugees, many with possessions piled on their heads, entered Uganda though the frontier district of Bundibugyo and had to sleep in school grounds under the stars or on shop verandas. The Red Cross said an estimated 2,000 of the refugees were pregnant women, and Uganda's New Vision newspaper reported that at least five gave birth while fleeing.

The Red Cross said on Monday it was working with Ugandan police to move hundreds of people from six reception centres to a transit camp where it could register them and offer help. "It's an enormous number of people," spokeswoman Catherine Ntabadde said.

"A lot of them walked long distances and we have to provide first aid. We need to provide food, shelter and medicine."

More people are arriving but others are starting to venture back, so the situation remains fluid, she added. The Uganda Red Cross will need to raise 2.5bn shillings (£640,000) for a three-month operation. The UN's world food programme is also taking part in the effort.The ADF launched a surprise attack on Kamangu last Thursday, briefly occupying it. The Ugandan military said Congolese troops had retaken the town.

Uganda has warned that the mass movement of people could allow rebels to slip in and launch attacks. Ugandan troops are screening the refugees to neutralise any possible ADF militia, worried that they might have gained expertise from al-Shabaab, the militant group operating in Somalia.

Paddy Ankunda, Uganda's military spokesman, told Reuters: "You can't be sure of the identity of each and every individual refugee and also the increasing volatility of the security situation right across the border worries us. Kamangu is only about 10km from the border.

"No doubt we've stepped up our security deployments along the border because we certainly can't pretend that everything is okay, but for now we're only monitoring events across the border. We haven't sent a single soldier into Congo."

The ADF waged an insurgency against Kampala in the late 1990s from its bases in the Ruwenzori Mountains and across the frontier in the eastern Congo jungle, including a 1998 attack in which 80 students were killed. A 2001 government offensive quelled the uprising and pushed its remnants deeper into eastern Congo. The group has kept a low profile since.

But a UN report last year said the rebels expanded their military capacity and co-operated with Somalia's al-Shabab militants. Uganda has said the buildup of the ADF could threaten its Lake Albert region, where oil reserves estimated at 3.5 bn barrels have been discovered, with production expected to commence soon.

The rebels' surprise attack last week came at a time when another group, the M23, had seized the attention of regional and international diplomatic efforts.

On Monday, fresh fighting erupted near Goma after more than 100 armed men disguised in women's clothing entered the country from Rwanda, according to residents.

Bifumbu Ruhira, a farmer at the village of Kanyarucinya on the frontline between the Congolese army and the M23 rebels, told Associated Press that the men got off two trucks and ran across the border. "They were wearing kikwembe (a shawl worn by Congolese women) over their uniforms, and women's headscarves."

A report published last month by the UN group of experts on Congo alleges that Rwandan soldiers have joined the M23 in recent months, a claim that Rwanda vehemently denies.