(CNN) -- Sudanese officials delayed for three hours the evacuation of three U.N. peacekeepers wounded in the contested region of Abyei by threatening to shoot down their helicopter, a U.N. official said Thursday.
The peacekeepers, all Ethiopians, were part of the U.N. Interim Security Force for Abyei (UNISFA). On Tuesday, they were patrolling Mabok, southeast of the town of Abyei, when their vehicle hit a landmine, the United Nations said in a report posted on the organization's website.
One soldier died immediately and three others -- who were waiting to be evacuated -- died later, said Alain Le Roy, the outgoing U.N. Under-Secretary-General for Peacekeeping Operations.
"We didn't get the clearance for the Medevac helicopter to take off immediately," Le Roy told reporters at U.N. headquarters. He cited a threat to shoot down the aircraft if it took off without clearance.
A board of inquiry is looking into the matter and Secretary-General Ban Ki-moon has raised the issue with the Sudanese authorities, Le Roy said.
He said that, under the status-of-forces agreement entered into by the world body and the government, medical evacuation flights do not require authorization.
Le Roy said he did not know whether the delay may have played a role in the three peacekeepers' deaths.
More than 1,500 troops -- all of them Ethiopian -- have been deployed in Abyei since June, when the Security Council voted to establish UNISFA after north-south violence resulted in more than 100,000 people fleeing their homes.
Abyei is a region under dispute between Sudan and the newly formed country of South Sudan, and was a battleground for decades in the brutal civil war fought between northern and southern forces. A referendum on whether the area should be part of the north or the South has been delayed over disagreement over who is eligible to vote.
The region, the size of Connecticut, is home to the Ngok Dinka people, who are closely allied with the South, but the area also serves as grazing grounds for northern Misseriya tribes.
CNN's Michael Martinez contributed to this report.