Two companies from Ghana and India were stopped by about 300 rebels from the Revolutionary United Front (RUF) at a location between Kenema and Daru.
The rebels were "tactically deployed" on high ground above the UN troops and were heavily armed, the spokesman said. Their weapons included rocket-propelled grenades.
The UN mission in Sierra Leone said it had contacted a former RUF officer, who is now a member of the Government and who happened to be in the area. Reportedly, he has gone to the scene in an effort to help resolve the stand-off.