The exact cause of the spill was unclear. Iranian oil and gas infrastructure has been under strain because of the U.S. blockade on the Strait of Hormuz, the narrow waterway into the Persian Gulf through which 20 percent to 25 percent of the world’s seaborne oil normally passes.

The Iranian government has also restricted ship traffic through the strait as talks on reopening the passage stall. And vessels and facilities have sustained damage in American and Israeli attacks, making them vulnerable to spills.

That has left tankers stranded, constraining exports and causing Iran to rapidly run out of places to store its oil, raising concerns of possible leaks or other mishaps at the Kharg Island hub. Large volumes of crude were being stored in tankers, adding to the risk of spills, said Dalga Khatinoglu, who follows Iran’s energy sector at Iran Open Data, an independent data initiative.

A rupture in an undersea pipeline connecting the hub with the Abuzar oil field, a major offshore field west of Kharg Island, was another possible source, Mr. Khatinoglu said. The poorly maintained, decades-old pipeline had suffered a number of leaks over the past several years, including a breach in October 2024, he said.

Others speculated that oil may have been deliberately discharged into the sea because of a lack of storage space, though there is no evidence for that. All told, “the naval blockade has likely pushed Iran’s oil system into a dangerous state,” said Nima Shokri, professor at the school of civil and environmental engineering at the Hamburg University of Technology. Shutting down oil wells is tricky, he said, because doing so can clog the wells or pipelines, or damage the oil reservoir underneath, making it slower and more expensive to restart production.

“Oil wells are not machines that can simply be switched off and restarted at will,” Dr. Shokri said.

As of midday on Thursday, the oil was drifting southward toward Saudi waters. Iranian state media has not reported on the slick. The Iran Ministry of Foreign Affairs did not immediately respond to a request for comment.

Keyvan Hosseini, an expert in energy and environment at the University of Southampton, said the spill reflected how sanctions, conflict and chronic underinvestment have made it much harder for Iran to modernize, maintain and replace critical oil infrastructure.

The Persian Gulf, much of which is shallow, is under increasing stress from heat, salinity, pollution and coastal development, he said. Oil can settle into sediments and shorelines and can be particularly damaging to mangroves, coral communities, seabirds, turtles and spawning grounds.

A spill near Kharg Island could affect fisheries, coastal communities, desalination plants, marine habitats and sensitive Persian Gulf ecosystems, Dr. Hosseini said. “Even a manageable spill can become a larger regional environmental crisis if the response is delayed,” he said.