MPONDWE, Uganda -- Militants linked to the Islamic State group massacred at least 41 people, mostly students, in western Uganda, the country's deadliest such attack in over a decade, officials said Saturday.

The military said it was pursuing the attackers from the Allied Democratic Forces, who also abducted six people during a raid on a school late Friday before fleeing back towards the Democratic Republic of Congo.

Officials and witnesses said guns and knives were used in a grisly late-night assault and dormitories set ablaze at Lhubiriha Secondary School in Mpondwe.

Police and army officials blamed the ADF, one of the deadliest militias over the border in DR Congo's strife-torn east, which the Islamic State group has called its local offshoot.

Sylvester Mapozi, the town council mayor of Mpondwe-Lhubiriha where the attack occurred, said 39 students were killed at the school.

"Within the community, as they (attackers) were going back, they also murdered two people, a female and a male. This is bringing up the number to 41," he said.

Many of the deceased were burned beyond recognition while other students were still unaccounted for, he said.

Mumbere Edgar Dido, 16, said the attackers arrived at his dormitory carrying machetes and guns and opened fire from outside, sending everyone diving under their beds.

"They continued to shoot through the windows, then set fire to our room while we were inside, before going to the girls' dormitory," he said.

'Appalling'

UN Secretary-General Antonio Guterres "strongly condemns" the attack, and sent his "heartfelt condolences" to the families of the victims, his spokesman said Saturday. "Those responsible for this appalling act must be brought to justice," Guterres' spokesman Farhan Haq said in a statement.

"Appalled by the attack at a secondary school in Western Uganda," wrote Molly Phee, the US assistant secretary of state for African Affairs, on Twitter.

"We send our condolences to the affected families and condemn the attack on schoolchildren."

France also condemned the deadly attack "in the strongest possible terms," a Foreign Ministry spokeswoman in Paris said Saturday.

Uganda Peoples' Defence Forces spokesman Felix Kulayigye said the six kidnapped were taken towards Virunga National Park, a vast expanse which straddles the border.

"UPDF embarked on pursuing the perpetrators to rescue the abducted students."

Police have not released details about the victims' ages or how many were students.

Outside the school, heavily armed soldiers and police stood guard, as a large crowd gathered and distraught survivors were comforted by loved ones.

It is the deadliest attack in Uganda since twin bombings in Kampala in 2010 killed 76 in a strike claimed by the Somalia-based Al-Shabaab group.

'Big attack'

According to a police report seen by AFP, police and military units were alerted to a "big attack" at the school around 11:00 p.m. local time on Friday evening.

"On arrival the school was found burning and the dead bodies of students were found lying in the compound and the school's food store broken into" with items missing, the report stated.

The school is less than 2 kilometers from the border with the DR Congo, where the ADF is mainly active. It has been accused of killing thousands of civilians there since the 1990s.

Major General Dick Olum told AFP that intelligence suggested the presence of the ADF in the area at least two days before the attack, and an investigation would be needed to establish what went wrong.

The attackers appeared to have detailed information about the school, he added.

"They knew where the boys and girls' dormitories resided," said Olum from Mpondwe, who has been leading an army operation against the ADF in Uganda and the DR Congo.

"This is why the rebels locked the boys' dormitory and set it on fire. The rebels did not lock the girls' section and the girls managed to get out, but they were cut with machetes as they ran for safety, and others shot."

'Killed brutally'

The rebels fled towards Virunga on the border with Uganda and Rwanda, a globally renowned sanctuary for rare species including mountain gorillas.

Militias -- of which dozens are active in the mineral-rich eastern DR Congo -- also use the park as a hideout.

Originally made up of mainly Muslim Ugandan rebels, the ADF gained a foothold in eastern DR Congo in the 1990s.

Since 2019, some ADF attacks in eastern DR Congo have been claimed by the Islamic State group, which calls the fighters the Islamic State Central Africa Province.

ADF attacks in Uganda are less common.

"They were killed brutally. ... It is pathetic and regrettable," Vice President Jessica Alupo told a graduation ceremony in Kampala Saturday. Florence Kabugho, an MP for Kasese, told reporters there were "very many unanswered questions" given the heavy military presence so close to the border.

Uganda and DR Congo launched a joint offensive in 2021 to drive the ADF out of their Congolese strongholds, but the measures have failed to blunt the group's violence.

It is not ADF's first attack on a school in Uganda.

In June 1998, 80 students were burnt to death in their dormitories in an ADF attack on Kichwamba Technical Institute near the DR Congo border. More than 100 students were abducted. (AFP)