The Conflict and Environment Observatory, an NGO that specialises in monitoring the environmental impact of conflict, identified a 20-kilometre-long oil slick on a satellite image taken by the European Copernicus program the day after the IRIS Dena was sunk, killing at least 84 sailors.

"We assess it to be highly likely that the spill originated from the vessel, based on the timeline of events and its correlation with reporting from the area," Leon Moreland, a researcher at CEOBS, told AFP on Wednesday.

The slick appeared to have drifted eastward, reaching protected marine and coastal areas two days after the attack, according to CEOBS and satellite imagery seen by AFP on Thursday.

The Sri Lanka Marine Environment Protection Authority (MEPA) launched cleanup operations on March 7 to remove hydrocarbon residues that had washed up on beaches on the southwest of the island.

Investigations conducted by MEPA found that the source of the oil slick was likely barrels of lubricant dumped at sea.

"This oil type has been identified as a piston-operated loose oil, not classified as a dangerous oil type," a MEPA spokesperson said at a press conference on March 8.

The oil slick was cleared by March 13, MEPA's chairman Samantha Gunasekara told AFP on Thursday.

MEPA has not reported any damage to marine fauna thus far.

Sri Lanka's coastal waters are home to rich biodiversity, including ecosystems -- such as coral reefs, seagrass beds, and mangroves -- that are particularly sensitive to hydrocarbon contamination.

"Local communities that rely on coastal fisheries may be indirectly affected through disruption to marine food chains and fish stocks," Moreland said.

The researcher said that regions affected by conflict face an elevated risk of oil spills.

The US and Israeli bombing of ports, oil depots, and vessels, along with retaliatory Iranian strikes on energy infrastructure in the Gulf and vessels near the Strait of Hormuz since the start of the Middle East war have increased the likelihood of "significant environmental incidents," Moreland added.