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By Aref Mohammed

BASRA, Iraq, March 24 (Reuters) - An Iraqi fisherman, who said he saw Iranian forces detain British sailors and marines in a waterway between Iraq and Iran, said on Saturday the ship British forces were searching was anchored in Iraqi waters.

Iran has condemned what it called the illegal entry on Friday of 15 British naval personnel into Iranian waters as a "suspicious act", the official IRNA news agency said.

Britain says they were detained in Iraqi waters on a routine search operation and has demanded their immediate return.

Brigadier Hakim Jassam of the Iraqi coastguard said it was not clear whether the ship was in Iraqi or Iranian waters.

The fisherman, a witness who alerted Reuters to the incident several hours before it was confirmed by the British government, said the line between Iranian and Iraqi waters was not marked by buoys but was well known.

"We've been working in this job for many years and because of our experience we can distinguish which is the Iraqi and which is Iranian side," he said, adding that Iraqi boats never venture across because of tight security by Iranian coastguards.

He said the ship the British forces were searching was among several that had been anchored for a week or more, waiting to unload or take on cargo at an Iraqi port.

"The ship was in Iraqi waters," said the fisherman, who spoke on condition of anonymity because of security concerns.

The Iraqi coastguard's Jassam said the incident happened at the entrance of the Shatt al-Arab waterway, where the open waters of the Gulf narrow into a channel that marks the southern border between the two countries.

"We don't know whether it happened in Iraqi or Iranian waters, we don't have exact information," Jassam told Reuters.

The detention has triggered a diplomatic crisis at a time of heightened tension over Tehran's nuclear ambitions.

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