Updated at 6:20 p.m., July 4

Police in the British territory of Gibraltar have seized an Iranian oil tanker suspected of carrying crude oil to Syria, violating European Union sanctions.

This is the first time an EU nation has taken such a bold move to keep sanctions intact.

Gibraltar authorities, aided by British Royal Marines, intercepted the Grace 1 early Thursday in Mediterranean waters claimed by both Britain and Spain, 4 kilometers off Gibraltar. They impounded the tanker.

"We have reason to believe that the Grace 1 was carrying a shipment of crude oil to the Banyas refinery in Syria," Gibraltar Chief Minister Fabian Piccardo said. "The refinery is the property of an entity that is subject to European Union sanctions against Syria."

Spain's foreign minister Josep Borrell told reporters the United States asked Gibraltar to seize the tanker.

White House national security adviser John Bolton did not say whether the U.S. made such a request, but welcomed the takeover, saying the U.S. and its allies will continue to stop Syria and Iran from "profiting off this illicit trade."

Reaction from Iran

Iran's foreign ministry has summoned the British ambassador to Tehran to express its "very strong objection to the illegal and unacceptable seizure" of its property.

Iran backs Syrian President Bashar al-Assad's fight against numerous rebel forces trying to topple him. Syria has taken back control of nearly the entire country except for Idlib province and scattered rebel strongholds.

The EU and United States have sanctions against Syria. President Donald Trump imposed sanctions on Iranian oil exports last year when he pulled the U.S. out of the 2015 nuclear deal with Iran.

Choking off Iran's market for oil has severely damaged the Iranian economy.