While the alert was sounded last week, traces of the fossil fuel have recently been spotted "on the ground" across an area of 70 to 100 kilometres (44 to 62 miles), between the towns of Mayumba and Mayonami in Nyanga province, the ministry statement said.

Videos posted on social media showed Mayumba's beaches covered with black trails. The ministry urged the public to temporarily limit activities including fishing, swimming and foraging in the affected areas due to the risk of contamination.

On social media, the Gabonese Agency for Aerospace Studies and Observations (AGEOS) said it had spotted two oil slicks covering an area of around 90 to 95 square kilometres (35 to 37 square miles) between late August and late September.

The spill could have been caused by a discharge from a ship or an offshore oil rig, AGEOS said.

Waste clean-up operations were planned, the environment ministry said. It did not specify whether those had already begun.