An oil spill off Vietnam has spread along more than 300 kilometers of coastline and affected popular tourist spots, but the source remains a mystery, an official said Monday. Thick, sticky blobs of oil have blackened beaches in the central provinces of Thua Thien Hue, Quang Ngai and Quang Tri, and authorities are investigating whether an offshore well could be to blame, said Nguyen Tran Manh, vice director of the National Center for Oil Spill Responses. The spill began affecting popular resort beaches along the coast last week near the ancient town of Hoi An, a UNESCO recognized site. Tourists were kept out of the water while the beaches were cleaned up. Government workers and soldiers were posted on popular beaches to clean up oil that continued to be blown ashore. Beaches in Danang have also been affected. Manh said tourist beaches have been cleaned, but large swaths of coastline remain blackened by the oil.