A video of the incident has been posted on social media.
There, two men in uniform are seen walking towards a UN convoy at the border crossing in Kasindi in northeastern Congo.
The UN force then opens fire before driving on and crossing the border.
The UN writes in a statement that "during the incident, soldiers from the peacekeeping force Monusco who returned from leave for an unclear reason opened fire at the border station and forced their way through".
Recently, Monusco's presence in the country has been met with wild protests.
UN peacekeeping forces have previously been criticized for failing to stop the fighting in the country's conflict-ridden east.