By Will Ross BBC Uganda reporter

The Ugandan army's struggle to defeat the notorious Lord's Resistance Army (LRA) of Joseph Kony faces a new challenge, after hundreds of LRA rebels crossed into northern Uganda.

The army, with support from the Sudanese authorities, has been in southern Sudan for over three months in an attempt to end the 16-year war which has seen thousands of young Ugandans abducted.

Last week the Ugandan army spoke of success at having killed dozens of rebels in an ambush.

But military sources say a group of 300 armed LRA rebels and 200 family members entered Uganda's Kitgum District at 0100 on Sunday.

They are under the control of Vincent Otti, one of Joseph Kony's most senior and brutal commanders.

They raided a trading centre at Pangira, in the Lokum Sub County of Kitgum District.

Food shortage

The Ugandan army then sent armoured vehicles to the area.

The raid on the trading centre provided an opportunity for several of the abducted LRA members to escape.

At least two women fled with their babies, and at least three armed rebels escaped.

They are now being debriefed by the Ugandan army and will benefit from the existing amnesty law.

Thousands of children have been abducted by LRA rebels

Reports from Kitgum District indicate that the rebels have been suffering in recent weeks due to lack of food and water.

The fact that the rebels raided the trading centre and have apparently not abducted more civilians suggests they crossed the border in order to replenish their supplies.

The remainder of the LRA rebels and captives are reported to be still inside southern Sudan with their overall commander, Joseph Kony.

On Monday afternoon military sources said the 300 rebels and 200 family members who crossed into Uganda were situated at Palabek, 40 kilometres northwest of Kitgum, and were proceeding into the north of Gulu district.

Embarrassment

The Ugandan army is currently pursuing those rebels.

For the past few weeks the Ugandan military was confident that, with the rebels holed up in the Imatong Hills of southern Sudan, it was only a matter of time before the LRA was defeated.

The fact that hundreds of rebels have managed to leave the hostile mountainous terrain and cross into Uganda must be embarrassing for the army, which has been claiming the Ugandan border is secure.

Meanwhile humanitarian agencies are becoming increasingly concerned about the plight of the several thousand LRA forced recruits and their children.

As a senior member of one international organisation put it: "The children are currently at more risk than ever before".

Efforts are also under way by humanitarian agencies to resurrect a peaceful means of ending the conflict.

Something the Ugandan army has so far ruled out.