Iran has said an investigation into the Stena Bulk tanker Stena Impero is still ongoing.

The foreign ministry confirmed this week that the detention order for the 50,000-dwt ship (built 2018) has been lifted, but the latest pronouncement seems to push back any prospect of its imminent departure from Iranian waters.

"The lifting of the detention order against Steno Impero was finalised today, but the investigation of some of its violations and environmental damage remains open," ministry spokesman Abbas Mousavi said in a tweet.