"The rebels...sprayed the vehicle with small arms fire, killing six people," army spokesman Lieutenant Paddy Ankunda told Reuters.
Ankunda said the attack occurred outside Kitgum, about 400 km (250 miles) north of Kampala.
"This ambush is unacceptable and not a good sign for peace, especially coming a day after the president extended the ceasefire offer," he said.
President Yoweri Museveni renewed a five-day ceasefire, which expired on Saturday, for five days more, starting on Monday.
The LRA, notorious for mutilating villagers and abducting children to fight in its ranks, has waged a 17-year-old war in northern Uganda against the government.
Museveni, who has pumped 14,000 troops into the area in the last 12 months along with tanks and artillery, offered the rebels a ceasefire in a last ditch attempt to end the rebellion.
- Reuters - Thomson Reuters Foundation
- For more humanitarian news and analysis, please visit https://www.trust.org/alertnet