WASHINGTON, Aug. 19 (Xinhua) -- U.S. Secretary of State Mike Pompeo on Monday called the release of the Iranian oil tanker "unfortunate," as the tanker seized by Britain for several weeks now has left Gibraltar.

"It's very unfortunate that that ship was released," said the top U.S. diplomat in an interview with U.S. Fox News.

Data from ship-tracking service MarineTraffic showed that the oil tanker set sail shortly before midnight on Sunday towards international waters and is heading for the Greek port of Kalamata, according to media reports.

Gibraltar earlier turned down the U.S. request to retain the tank carrying 2.1 million barrels of oil, which Washington said was "intrinsically linked" to the U.S. sanctions against Iran. Legally, U.S. sanctions only apply between the United States and Iran, and no one else.

In his interview on Monday, Pompeo claimed that if Iran was successful in making profits by selling the oil from the tanker, Iran's elite forces will "have more money, more wealth, more resources to continue their terror campaign, to continue their assassination campaign in Europe."

An anonymous U.S. State Department official told media on Monday that Washington has expressed its "strong position" to Greek authorities and all ports in the Mediterranean.

The official said assisting the Iranian tanker could be viewed as material support to a U.S.-designated foreign terrorist organization, according to U.S. media reports.

The Iranian Foreign Ministry on Monday warned against any attempt by the United States to seize the Iranian oil tanker.

"Any attempt to do this (seize the Iranian vessel) ... would be a threat to the free maritime shipping," Iranian Foreign Ministry Spokesman Abbas Mousavi said.

The oil taker was detained in July off the coast of the British Overseas Territory of Gibraltar by Royal Marines on suspicion of attempting to carry oil to Syria "in breach of EU sanctions," charges denied by the Iranian government.