Last updated on Aug. 18, 2019 at 8:30 p. m.

WASHINGTON - An Iranian supertanker left Gibraltar on Sunday after authorities there rejected a U.S. bid to detain it, but it was unclear where the ship might be headed next.

Marine traffic monitoring data showed the tanker leaving Gibraltar's waters. Both Iran and Gibraltar confirmed the move.

Tehran said it was ready to dispatch its naval fleet to escort the ship, loaded with 2.1 million barrels of light crude oil worth $130 million, but Iran gave no indication where it would set sail for.

The ship, called the Grace 1 but now renamed by Iran as Adrian Darya 1, was seized July 4 by Gibraltar, an overseas British territory, because authorities there believed the crude oil was headed to Syria, an Iran ally, in violation of European Union sanctions. Originally, the ship was flying under a Panamanian flag but after it was renamed, a red, white and green Iranian flag was hoisted over the ship.

The ship's seizure was one of several related incidents in recent weeks triggering increased tensions between Tehran and Western nations. Later in July, Iran seized a British-flagged oil tanker, the Stena Impero, in the Persian Gulf and is still impounding it.

The United States and Iran have shot down each other's unmanned drones, and Western countries have accused Tehran of carrying out other attacks on ships in the Gulf, where a fifth of the world's oil production passes through the Strait of Hormuz.

The incidents stem at least in part from U.S. President Donald Trump's withdrawal last year from the 2015 international nuclear agreement aimed at restraining Tehran's nuclear weapons program. Trump then reimposed debilitating sanctions, which have hobbled the Iranian economy.

Gibraltar authorities on Thursday decided to release the Iranian tanker, saying they had received written assurances from Tehran that the crude oil would not be shipped to Syria.

On Friday, the U.S. government won a court order in Washington authorizing the seizure of the ship, the oil it carries and nearly $1 million. The U.S. contended that Iran's Islamic Revolutionary Guard Corps, listed as a terrorist group by Washington, was making the illegal shipment to Syria in violation of the U.S. sanctions against Iran.

But Gibraltar said Sunday it "is unable to seek an order of the Supreme Court of Gibraltar to provide the restraining assistance required" by the United States.

The Gibraltar government said the European Union sanction "against Iran - which is applicable in Gibraltar - is much narrower than that applicable in the U.S."