It reported eight minor injuries to force members and damage to vehicles and property. There had been a news report of an injury to an Islamist Bedouin, but the MFO said intruders left "with no known injury." The situation stabilized after Egyptian authorities "helped secure the camp's perimeter."
Militants have threatened the observers before, saying they would attack if they didn't leave.
"The MFO views these events with the gravest concern and is coordinating with Egyptian authorities regarding strengthened measures to secure MFO premises and personnel, and to ensure that the MFO mission may continue securely and effectively," it said.
Egyptian security has been fighting militants in Sinai since August. That's when 16 border guards were killed in an attack by Islamist militants.