The mission's deputy chief, Joel Adechi, told reporters that the expulsions would affect all aspects of mission operations, including supplies, transport, finance and communications.
Eritrea issued the order on Wednesday, giving U.S., Canadian, European, and Russian members of the mission 10 days to leave.
The letter gave no reason, but Eritrea has complained the United Nations is not applying enough pressure on Ethiopia to accept an independently-drawn final border.
The countries fought a border war from 1998 to 2000 that killed 70,000 people.
Both the U.N. Security Council and Secretary-General Kofi Annan have rejected Eritrea's expulsion order, saying it violates Eritrea's obligations under the U.N. Charter.