Iran impounded the Stena Impero vessel on July 19 in the Strait of Hormuz for allegedly breaking international maritime rules, causing tensions with Britain as it made a proposal for naval missions to escort ships in the Persian Gulf.
The sailors - five Ind, one Latvian and one Russian - had already left the Stena Impero, the BBC quoted a foreign ministry spokesman as saying.
Iranian foreign ministry spokesman Abbas Mousavi said the ship's captain had selected seven members of its crew to be released.
"They have left the vessel and the final procedures are underway to send them back to their countries," he said, adding that the decision had been made in line with Iran's "humanitarian policies".
"The consular status and individual permits of the crew were reviewed, and after further consideration and also for humanitarian grounds, some of the crew were allowed to leave the ship and return to daily life," Efe news quoted Mousavi as saying.
He stressed the Iranian authorities had "no problem with the crew and the captain" and at issue were "violations that the vessel committed".
He did not say what would happen to the remaining 13 Indian, two Russian and one Filipino crew members.
The UK has said Iran had no right to obstruct the vessel's passage and accused it of an "act of state piracy".
The Stena Impero's seizure came after the British Royal Navy patrol vessel guards on July 4 stopped the Iranian oil supertanker Grace 1 off the coast of Gibraltar on suspicion of carrying oil to Syria, a country subject to European Union sanctions.
The Iranian tanker was freed on August 15 by order of a court in Gibraltar, a British overseas territory on the southern tip of Spain, after Iran provided assurances that the tanker's cargo would not be taken to Syria.
( With inputs from IANS )