The United Nations says a board of inquiry is looking into the deaths of three wounded Ethiopian peacekeepers who died in Sudan when authorities delayed their evacuation by helicopter.

U.N. peacekeeping chief Alain Le Roi told reporters medical flights are not supposed to require authorization under the peacekeeping agreement between the U.N. and Sudan.

But he said the crew of the Medevac helicopter complied with the delay because Sudanese troops threatened to shoot them down if they took off without clearance.

Le Roi said there is no way to know whether the three peacekeepers, critically injured in a land mine blast, would have survived if they had been moved sooner.

A U.N. spokesperson said Secretary General Ban Ki-moon had taken the issue to Sudan's ambassador to the U.N. The spokesperson said Mr. Ban stressed that any delay is “unacceptable” when it comes to saving lives.

The three peacekeepers were wounded Tuesday in a blast while on patrol in the disputed Abyei region. A fourth peacekeeper died instantly in the explosion. Seven others were wounded but survived.

Ethiopia provided the troops for the recently-approved Abyei peacekeeping force.

Abyei sits on the border between Sudan and the new nation of South Sudan. The two countries have been unable to decide which one should control the oil-rich region.

Sudanese troops occupied Abyei on May 21, about seven weeks before South Sudan declared independence.

Both sides have agreed to withdraw their forces under a deal that led to the creation of the U.N. peacekeeping force.