A suspected oil slick off Iran’s Kharg Island appears to be smaller than initially feared, a UK environmental group said Saturday, with satellite images showing it “much reduced” from earlier in the week.

The cause remains unknown, though some experts have pointed to potential strain on Iran’s oil storage infrastructure amid a U.S. naval blockade.

What do satellite images show about the Kharg Island oil slick?

The Conflict and Environment Observatory, a U.K.-based NGO, said Saturday that the suspected slick appeared “visually consistent with oil” based on analysis of imagery from the Copernicus Data Space browser, but that its cause and origin “remain unknown and cannot be determined conclusively from the available imagery alone.”

Copernicus images on Saturday appeared to show the slick was “much reduced” from the first pictures seen on Wednesday.

The observatory put the spread of the original slick at about 44 square kilometers, while Orbital EOS, which monitors oil spills, told The New York Times the spill appeared to cover more than 52 square kilometers as of Thursday.

What may have caused the suspected oil spill near Kharg Island?

Leon Moreland of the Conflict and Environment Observatory said offshore infrastructure in the wider area could be a potential source, but that no definitive point of origin could be identified.

Some media reports, including by Fox News, suggested Iran’s oil storage facilities may be coming under strain as a U.S. naval blockade disrupts the country’s ability to export or store crude.

Moreland added there was “no obvious evidence of additional active spills around the island, although the earlier slick can still be observed moving south.”

What did Iranian officials say about the oil spill reports?

Iranian officials pushed back on reports that the spill was linked to strained storage facilities. Moussa Ahmadi, head of the Iranian parliament’s energy commission, told ISNA news agency Saturday there was “no official report so far confirming” that Iran’s oil facility was leaking.

“Production in various oil fields in the country continues incessantly and without any problem,” he said. Jafar Pourkabgani, a lawmaker representing the coastal city of Bushehr, dismissed reports as “false,” saying the stains seen in satellite images were oil and ballast water waste dumped by a European oil tanker.

Why is Kharg Island critical to Iran’s oil industry?

Kharg Island sits off Iran’s Gulf coast, hundreds of kilometers northwest of the Strait of Hormuz, and serves as the primary export terminal for Iranian crude oil.

It is a key pillar of Iran’s economy, which has come under severe strain since the country’s conflict with the U.S. and Israel began Feb. 28.

Iran largely closed the Strait of Hormuz at the start of the conflict, prompting the U.S. to impose a blockade of Iranian ports. Many tankers remain stranded in the area as a result of both blockades.