Iran has freed nine of 12 Indian crew from a Panama-flagged tanker seized on July 14, India's foreign ministry said Friday.

Iran had accused the MT Riah ship of smuggling contraband fuel when it was detained, amid mounting tensions between the Iranian government and Britain and the US over shipping in the Strait of Hormuz.

The TankerTrackers online oil shipment website reported that the MT Riah crossed into Iranian waters on July 14 and stopped transmitting signals shortly after.

India's foreign ministry did not give details of the release of the nine men.

It left 21 other Indians in Iranian detention however, including three others from the MT Riah and 18 from the British-flagged Stena Impero tanker which was captured by Iranian forces last week.

The Stena Impero and its 23 crew have been impounded at the southern port of Bandar Abbas for allegedly breaking "international maritime rules". The vessel is at the heart of the showdown between Iran and Britain.

Apart from the 18 Indians, there are three Russians, a Latvian and a Filipino on the ship.

India announced on Thursday that its diplomats in Iran had been given access to Stena Impero crew.

"All 18 Indian crew members on board are safe and doing fine. Will continue to push for their early release," junior foreign minister V Muraleedharan said on Twitter.

Images from inside the ship released by Iran on Monday showed some Indian crew sat around a table, chatting and smiling. Two members could be seen cooking in the ship's kitchen.

The tanker's seizure has deepened a crisis triggered between Iran and Western countries in April when the United States said it would sanction countries buying oil from Iran.

Iran has hinted it is open to swapping the Stena Impero for an Iranian vessel, Grace 1, that was detained in Gibraltar allegedly carrying Iranian oil to Syria in breach of international sanctions.