DUBAI, May 12 (Reuters) - Iran's top environmental official said on Tuesday that a suspected oil spill in the Gulf near Iran's Kharg Island was likely caused by a tanker dumping waste water and not a leak from oil facilities.

The suspected oil spill covering dozens of square kilometres of sea near Iran's main oil hub of Kharg Island has been seen on satellite imagery.

The likely spill - appearing on images as a grey and white slick - covered waters to the west of the 8-kilometre (5-mile) long island, pictures from Copernicus’s Sentinel-1, Sentinel-2 and Sentinel-3 satellites showed on May 6-8.

But Iranian Vice-President Shina Ansari said: "Our monitoring results show that this spill was caused by the discharge of ballast water contaminated with substances from a non-Iranian tanker, and no oil leaks have been reported from (Iran's) pipelines or oil facilities," state media reported.

Louis Goddard, co-founder of consultancy Data Desk, which focuses on climate and commodities, said earlier that the images likely showed an oil slick, which he said was potentially the largest to occur since the start of the U.S.-Israel war against Iran 70 days ago.

Iran’s Oil Terminals Company said on Sunday inspections had found no evidence of leaks from storage tanks, pipelines, loading facilities or tankers operating near the island.