It was not immediately clear what had caused the apparent spill, which was located off the small Gulf island's west coast.

Orbital EOS, which monitors oil spills, told The New York Times that the spill appeared to cover more than 20 square miles (52 square kilometers) as of Thursday.

The Conflict and Environment Observatory, a non-governmental organization, said on X that the "original source remains unclear, meanwhile it's drifting south and seems unlikely to be addressed appropriately."

Kharg Island is at the heart of Iran's oil export industry, a lynchpin of the country's battered economy. It sits off Iran's Gulf coast, hundreds of kilometers northwest of the narrow, strategic Strait of Hormuz.

Iran largely closed the strait at the start of its conflict with the United States and Israel on February 28. The United States has since imposed a blockade of Iranian ports, stranding many tankers in the area.

Kharg Island has Iran's largest oil terminal, oil pipelines, storage tanks and related infrastructure.