By Elias Biryabarema

KAMPALA (Reuters) - Militants linked to Islamic State killed 37 people and abducted six others in an attack on a school in western Uganda near the border with the Democratic Republic of Congo, the military said on Saturday.

Military personnel found the bodies of the dead when they arrived at the school, defence spokesperson Felix Kulayigye said in a statement.

"Our forces are pursuing the enemy to rescue those abducted and destroy this group," he said earlier on Twitter. The attackers, from the rebel group Allied Democratic Forces (ADF), fled towards Virunga National Park in Congo, police said.

Police said eight other people were in hospital with critical injuries after the attack at the Lhubirira Secondary School in Mpondwe.

Privately-owned NTV Uganda television said on Twitter that the death toll stood at 41, while state-run New Vision newspaper said it was 42. New Vision said 39 of the dead were students, and that some of those killed died when the attackers set off a bomb as they fled.

Neither police nor military said how many of the dead were schoolchildren.

The assailants, numbering about five, burnt down a dormitory and looted food, police and the military said.

Major General Dick Olum, the army's commander for western Uganda and in charge of a military deployment in the Democratic Republic of Congo, said the attackers had stayed in the town two days before the attack, marking their target.

He said an unidentified youth had gone to the school to check its layout before the attack.

"That is how the attackers came and locked the boys' door. The boys really tried to fight back, but they were overpowered. The attackers had lit mattresses," Olum told reporters from Mpondwe, according to a video posted on Twitter by the Daily Monitor newspaper.

"In the girls dorm, they found their door open, hence killing them and cutting them."

The ADF rebels launched their insurgency against President Yoweri Museveni in the 1990s from an initial base in the Rwenzori Mountains.

The group was largely defeated by the Ugandan military but remnants fled across the border into the vast jungles of eastern Congo from where they have since maintained their insurgency - perpetrating attacks on civilian and military targets in both Congo and Uganda.

In April, the ADF attacked a village in eastern Democratic Republic of Congo, killing at least 20 people.

(Reporting by Elias Biryabarema; Additional reporting by Jose Joseph in Bengaluru and George Obulutsa in Nairobi, Fiston Mahamba in Goma; Editing by William Mallard and Frances Kerry)