LAGOS -- The United Nations is suspending aid to dangerous areas of Nigeria's northeastern Borno state, where it says a half million people are starving, after Boko Haram ambushed a humanitarian convoy.

Three civilians including a UNICEF employee and contractor for the International Organization for Migration were wounded in Thursday's ambush, along with two of the soldiers escorting the humanitarian workers, according to the Nigerian army and the U.N. Children's Fund.

"Only the U.N. missions outside the capital have been suspended," UNICEF spokeswoman Doune Porter told The Associated Press on Friday. "The normal assistance we have been giving will continue in Maiduguri," the Borno state capital of 1 million people that hosts another million refugees from Nigeria's 7-year-old Islamic insurgency.

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"This was not only an attack on humanitarian workers. It is an attack on the people who most need the assistance and aid that these workers were bringing," Porter said.

The attacked convoy was traveling from the city of Bama, newly freed from Boko Haram, where Doctors Without Borders has warned that children are dying every day with 15 percent suffering severe acute malnutrition and likely to die without food and medical aid.

More than 500,000 people are suffering a "catastrophic humanitarian crisis" in dangerous-to-reach areas, said the doctors.

A Doctors Without Borders vehicle traveling with a military escort set off a land mine earlier this week a few miles from the scene of Thursday's ambush but no one was hurt, according to soldiers who were there. They spoke on condition of anonymity because they are not authorized to speak to reporters.

Army spokesman Col. Sani Kukasheka Usman said the insurgents were hiding in Meleri village near Kawuri, the official gateway to the sprawling Sambisa Forest that has been a Boko Haram stronghold. The military warned earlier this month that Boko Haram fighters were fleeing its daily aerial bombardments and ground attacks in the forest, heading toward the border with Cameroon.

Some Boko Haram fighters who surrendered have reported the militants are running out of food, fuel and arms, the military has stated, saying they have cut off the insurgents' supply lines.

The uprising by Boko Haram, which joined the Islamic State group last year, has killed more than 20,000 people, forced more than 2 million from their homes and spread across Nigeria's borders to Cameroon, Chad and Niger.