Satellite imagery has revealed signs of an oil spill off the coast of Kharg Island, Iran’s main oil export hub. The images show a visible oil slick spreading across the sea near the island’s western shore. The exact cause of the spill remains unconfirmed, according to reports from AFP.

Monitoring organization Orbital EOS told The New York Times that as of Thursday, the slick covered about 20 square miles (52 square kilometers). The nonprofit Conflict and Environment Observatory stated on social media that the source of the spill is still unclear, noting that the slick is drifting southward and that effective containment measures appear unlikely.

Kharg Island hosts Iran’s largest oil terminal, pipelines, storage tanks, and related infrastructure. The island lies several hundred kilometers northwest of the strategic Strait of Hormuz, which Iran nearly closed after conflict with the United States and Israel began on February 28. Subsequent U.S. blockades of Iranian ports have left many oil tankers stranded in the area.