Britain has said that investigations into an Iranian oil tanker its forces captured near Gibraltar was a matter for the government of the British overseas territory to deal with, after an Iranian news agency reported that the vessel would be released on August 13.

British Royal Marines seized the Grace 1 supertanker on July 4, accusing it of violating EU sanctions on Syria, which Tehran denies.

'The investigations being conducted around the Grace 1 are a matter for the government of Gibraltar,' a British Foreign Office spokesman said. 'As this is an ongoing investigation, we are unable to comment further.'

Earlier, a senior Gibraltarian government source denied Iran's semiofficial Fars news agency report that the tanker would be freed on August 13 in the evening.

The source, who spoke on the condition of anonymity, said that report was not correct.

A spokesman for Gibraltar said, 'We continue to seek to de-escalate issues arising since the lawful detention of Grace 1.' The current detention order on the vessel expires on August 17 in the evening, the spokesman said.

Tensions have risen between Iran and the West after U.S. President Donald Trump pulled out of a 2015 landmark nuclear agreement which curbed Tehran's atomic program in return for an easing of economic sanctions.

Two weeks after the Grace 1 was detained, Iran's Islamic Revolutionary Guards Corps seized British tanker Stena Impero near the Strait of Hormuz for alleged marine violations.

Also on August 13, Iran said it has enriched 370 kilograms of uranium to a concentration of 4.5 percent.

Behrouz Kamalvandi, a spokesman for the Atomic Energy Organization of Iran, made the claim to the Iran's ISNA news agency.

Both the amount and the enrichment level would be in violation of the 2015 deal.

The 2015 agreement allows Iran to enrich 300 kilograms of uranium to a level no higher than 3.67 percent.

Iran warned over the past months that it would gradually withdraw from the deal after Washington pulled out and reimposed tough sanctions.

European powers, who opposed the American move, have sought to convince Iran to remain in the deal.

With reporting by Reuters and dpa

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