Stena Impero has been held offshore near Iran's southern port of Bandar Abbas (File Photo)

Some of the crew of a Swedish-owned oil tanker held by Iran in the Gulf have been released, the Swedish foreign ministry said on Wednesday, weeks after it was taken amid heightened tensions in the region.

Iranian forces seized control of the British-flagged Stena Impero on July 19 as it was navigating through an international passage in the Strait of Hormuz at the entrance of the Gulf.

Tehran denied the seizure was a tit-for-tat move after British commandos seized an Iranian oil tanker on July 4 as it passed through Gibraltar's waters, under suspicion it was breaking EU sanctions on oil deliveries to Syria.

"A part of the crew of the Stena Impero... has been released," the Swedish foreign ministry said in a message to AFP without giving details of the how many had been freed.

Earlier Wednesday, the chief executive of Stena Bulk that owns the tanker said he expected seven of the crew to be released later in the day. The tanker has a total crew of 23 on board.

"We view this communication as a positive step on the way to the release of all the remaining crew, which has always been our primary concern and focus," Stena Bulk said in a statement.

The ship has been held offshore near Iran's southern port of Bandar Abbas.

Iranian officials have given varying reasons for its seizure and continued detention.

Some, such as Foreign Minister Mohammad Javad Zarif, said it was for breaching maritime regulations.

It has also been claimed that its transponder was turned off, it was going the wrong way along a shipping channel or had collided with an unidentified fishing vessel.

The Iranian oil tanker seized by British commandos, the Adrian Darya 1 (formerly the Grace 1), was ordered released by Gibraltar on August 15 and was last registered off the coast of Lebanon and Syria on Monday.