Singapore has expressed "deep concern" over reports of Iran firing on one of its ships in international waters.
Its marine and port authority said it was an "interference with navigational rights" and a "serious violation of international law".
US officials said Iranian forces fired warning shots at the Singapore-flagged Alpine Eternity, operated by Norway's Transpetrol TM AS, last Thursday.
The ship collided with an Iranian rig on 22 March in the Gulf.
It is now out of Iranian waters and in Dubai. Singapore said Alpine Eternity had reported the collision to its port authority, and the city-state was investigating.
Transpetrol and the ship's owner South Maritime have said previously that the rig was uncharted.
The Iranian oil ministry's news service Shana said the ship had drifted off course and caused $300m (£191m) in damage in the collision, which caused a "very dangerous situation" for the affected oil wells.
It added that Transpetrol had not taken steps towards compensation, and that it wanted neighbouring countries to detain the ship.
Reuters quoted an Iranian official as saying on Shana that the ship "was trying to leave the region before the issue could be settled."
Transpetrol's spokesman Pat Adamson told the BBC that the company was in "continuing dialogue" with Iranian authorities.
He said the ship needed repairs after the collision and went to a dry dock at Dubai. It was heading to the port of Fujairah to await instructions for its next load when it was shot at by small boats, and was now back in Dubai, said Mr Adamson.
He said its insurers had encountered difficulties in posting a bond during negotiations, because of international sanctions against Iran.
But he said the company and insurers were in talks with the US and the UK Treasury to be allowed to do so, and that they were "close to succeeding".
"We are extremely disappointed with the actions taken, as the shots fired posed potential danger to human life," he said.