SIX divers were trapped 70 metres (77 yards) below the surface of the sea overnight after a ship carrying 72 people sank in bad weather off the coast of Iran.

The diving support vessel Koosha 1 is thought to have sunk at around 5:30pm local time Thursday, off the Persian Gulf island of Lavan.

Rescue teams saved 60 lives, while five people drowned and one is unaccounted for, according to the semi-official Fars news agency.

The vessel apparently went down nose first with such speed that the crew did not have chance to issue a distress signal.

High winds were hampering efforts overnight to save the trapped saturation divers, who have been underwater in a hyperbaric recompression chamber that is bolted onto the deck for more than 18 hours.

Their survival depends upon the seals remaining intact and a continuing supply of the mix of oxygen, nitrogen and helium that they breathe.

The Koosha 1 is owned by the Iranian marine salvage group Darya Koosh. It is believed that the team planned to recover an abandoned crane and other machinery from the seabed.

Nigel Dixon, the diving manager of Dubai-based Dulam International, which is leading the rescue operation, said the rescue team was searching for the wreck using sonar but was faced with bad weather.

"The wind is 30 knots, and it's blowing like hell out there," he said. "It does not look good. They have been down there for 18 hours without life support, so CO2 levels will be very high. The chamber was only pressurized to 60 metres, so there is also a danger that the seals may have ruptured."

He added, "The divers could not be evacuated in time -- the accident happened instantaneously. There is an oil slick at the location, which we believe is from the wreck. We will send down our own dive to try and retrieve the chamber."

Originally published as Divers trapped on sea bottom after ship sinks