The pipeline pumps oil to the nearby central command of NATO's military forces.
The accident occurred late on Wednesday. The leak was eventually plugged at 4:00 a.m. local time (02:00 GMT), RTBF TV reported. A special commission has been set up to assess the environmental damage caused.
Local officials said the accident did not pose a threat to local residents as most of their houses were far away from the scene of the accident. However, Belgian authorities have asked people not to use water from nearby wells.