"The collision between USS Philadelphia and the Turkish-flagged MV Yaso Aysen occurred at approximately 2 am local time (2300 GMT Sunday) while the submarine was conducting surfaced operations as it transited to Bahrain for a scheduled port visit," said a statement from the US Fifth Fleet in Bahrain.

It said the submarine's propulsion system was unaffected by the accident but that the vessel would head to Bahrain to check for any exterior damage.

"The incident is currently under investigation," it added.

The 6,000-ton USS Philadelphia, an attack submarine which is powered by a nuclear reactor, is involved in maritime security operations in the region, including counter-terrorism efforts.

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