The U.S. government has applied to halt the release of an Iranian oil tanker detained in Gibraltar last month, according to the government of British overseas territory.

Grace 1, the Iranian oil tanker, was impounded by the Royal Marines on July 4 off Gibraltar. Reports said a deal was being worked out to release the tanker in return for the release of a British tanker seized by Iran, but the U.S. applicaiton to take control of the tanker in the last minute will scupper any such deal. The U.S. request will be considered at a court hearing at 15:00 BST.

“U.S. Department of Justice has applied to seize the Grace 1 on a number of allegations which are now being considered,” Gibraltar officials said in a statement to NBC News.

A spokesperson for the Gibraltar government had confirmed that the way was being cleared to release the ship and the police proceedings against the four crew members had ended.

A court in the British overseas territory of Gibraltar is to decide on Thursday on the fate of supertanker Grace 1, seized with the help of British Royal Marines on July 4 in an operation that ratcheted up tensions with Iran A court in the British overseas territory of Gibraltar is to decide on Thursday on the fate of supertanker Grace 1, seized with the help of British Royal Marines on July 4 in an operation that ratcheted up tensions with Iran Photo: AFP / JORGE GUERRERO

However, intervention from the U.S. has forced them to reconsider the release.

Grace 1 was captured last month, after it was suspected of shipping oil to Syria in violation of European Union sanctions. The Royal Marines had stormed the ship and taken control of it on a request by the Gibraltar government. The ship's capture had led to a standoff with Tehran. Iranian Revolutionary Guard then seized the British-flagged Stena Impero in retaliation. The ship is still moored in an Iranian port.

The crew members, which includes people of Indian, Russian, Latvian and Filipino nationalities, have spent 43 days in detention abroad the ship.