A British Royal Navy ship (back R) patrols near supertanker Grace 1 suspected of carrying crude oil to Syria in violation of EU sanctions after it was detained off the coast of Gibraltar on July 4, 2019. (AFP photo)

TEHRAN, July 4 (Xinhua) -- Iran's Foreign Ministry on Thursday summoned British ambassador to Tehran, Robert Macaire, following its oil tanker seizure by Britain in the Strait of Gibraltar.

The Iranian Foreign Ministry called the seizure of Iranian oil tanker as "illegal."

"We declared to the ambassador that this move amounts to strange unconventionality, because the sanctions that they have announced are not based on the UN Security Council (resolutions) and the Islamic Republic of Iran does not accept this measure (seizure of the tanker)," Iran's Foreign Ministry spokesman Abbas Mousavi told state TV.

Earlier on Thursday, British Royal Marines helped the authorities in Gibraltar seize the ship which was heading to Syria "in breach of EU sanctions," Western media reported.

Spain's acting foreign minister said the seizure of the ship "Grace 1" was at the U.S.'s request.