Armed rebels attacked a school in western Uganda, killing at least 41 people, mostly students, and abducting six others, Ugandan officials have said.

Some were hacked to death with machetes while others died when their dormitories were set on fire, military spokesman Felix Kulayigye told CNN.

About 20 members of the ISIS-linked Allied Democratic Forces (ADF) rebel group attacked the Lhubirira secondary school late on Friday, according to Uganda's military.

The school is situated along Uganda's border with Congo in the town of Kasese, and educates children between the ages of 13 and 18. Of the dead, 39 were students and two were from the local community, local officials said.

Authorities were still trying to extinguish the fire by Saturday morning. The military suspects more dead bodies may be found, but said there was no one still alive trapped in the school.

According to Kulayigye, there had been 62 people in the school during the attack.

"We're commiserating with the families, and the leadership of the UPDF (Ugandan Peoples' Defense Forces) are on the ground and have been deployed to protect the place," he added.

The spokesman for Uganda's military operations in the Democratic Republic of Congo (DRC), Major Bilal Katamba, told CNN the military is pursuing the suspects in the DRC and believes they are headed towards the Virunga National Park.

Major Dick Olum, commander of the UPDF operation in eastern DRC, said the rebels had spent two days in the town -- where they were led and shown around by local residents -- before the killings.

"Everybody should be on the lookout," he warned residents in Kasese.

"If you see someone you don't recognize, have him arrested. Please identify the youth that led the ADF here," he appealed.

'Very crowded' mortuary

An eyewitness who lost three cousins in the brutal attack told CNN that the local mortuary is "very crowded" as victims rescued from the school continue to be treated.

"I knew my relatives were at school, so when I heard about the incident, very early in the morning we rushed to the hospital and we found their bodies there, and the mortuary was very crowded," Clay Biromunane said.

Biromunane was in his bedroom around 300 meters away from the school when he heard gunfire erupt at around 10:40 p.m. local time.

"Up to now people are up and down looking for their relatives," Biromunane said.

The 35-year-old said the community was "very much surprised" by the attack and said they had never experienced anything like it before.

Mapoze Slevest, Mayor of Mpondwe, similarly expressed surprise, telling reporters that Mpondwe "was a good community with no rebels."

Based along the mountainous border between Uganda and the DRC, the ADF began fighting against the government after being founded in the mid-90s.

The group was sanctioned by the United States and the United Nations in 2014 for terrorist activities in the region, including attacks on children.

The US Department of Treasury denounced the ADF "for targeting children in situations of armed conflict, including through killing, rape, abduction and forced displacement."

In January, the ADF was accused of detonating a bomb during a church service in the DRC, killing at least 12 worshipers.