An Iranian oil tanker held for six weeks after being impounded left Gibraltar on Sunday, after the authorities in the British territory rejected a request from American officials to turn the vessel over to them, the latest development in an episode that has contributed to raised tensions between Iran and the West.

A marine traffic monitoring website showed the tanker, the Grace 1, leaving Gibraltar’s waters shortly after 11pm (local time), with a destination of Kalamata, a port city in southern Greece, a move that has raised hopes that Iran will now release a British-flagged ship.

The Iranian vessel is expected to arrive in Greece on Sunday, according to the tracker. News organisations in Iran and Gibraltar confirmed that the ship had left Gibraltar, and Iran issued a warning soon afterward that any attempt by the US to detain the tanker would have consequences.

The ship was renamed Adrian Darya-1, with its new moniker hastily painted onto the side of the vessel in white lettering seen in images taken shortly before it left the port, with “Grace 1” painted over in black.

The ship was seized on July 4 by British marines and Gibraltar port officials who asserted that the tanker was carrying oil to Syria in violation of a EU embargo. Iran soon detained a British-flagged tanker, the Stena Impero, in the Strait of Hormuz, the narrow entryway to the Persian Gulf that is a conduit for about 20 per cent of the world’s crude oil.

The decision to release the Iranian ship last week was seen as a sign of easing of tensions between Gibraltar, a semiautonomous British territory, London and Tehran. A confrontation between Iran and the West, particularly with the US, has escalated in recent weeks.

The ship’s departure also raised expectations that Iran, in turn, would relinquish the Stena Impero.

Gibraltar ordered the release of the Grace 1 on Thursday despite a last-minute request from the US that it be allowed to seize the ship. The following day, the justice department unsealed a warrant issued by a federal court in Washington to seize the tanker, the oil it contains and nearly $1 million based on statutory violations.

“A network of front companies allegedly laundered millions of dollars in support of such shipments,” Jessie Liu, the US attorney for the district of Columbia, said. Multiple parties affiliated with the Islamic Revolutionary Guards Corps of Iran, which the US has designated a terrorist group, were believed involved.