Iran's Supreme Leader Ayatollah Ali Khamenei said on Tuesday that Britain's "illegal seizure" of an Iranian oil tanker will not go unanswered, the state TV reported.

"Britain commits an act of maritime piracy and steals our ship," Khamenei was quoted as saying.

Iran will not leave this "wickedness" unanswered and will respond to it at an appropriate time and place, he added.

The British military seized Iranian oil tanker Grace 1 off the coast of Gibraltar earlier this month on suspicion of carrying crude oil to Syria "in breach of" the European Union's sanctions.