Iran says the seizure of a super-tanker off the coast of Gibraltar last week will not go "unanswered".
The Iranian military's chief of staff, Major General Mohammad Bagheri says they will respond "when necessary" and claims the capture of the Grace 1 was based on "fabricated excuses".
Last week, Iran described the seizure as "piracy, plain and simple".
Gibraltar's government says the vessel was "loaded to capacity" with crude oil.
Royal Marines boarded the Grace 1 off Gibraltar's coast on 4 July - on suspicion of breaching EU sanctions against Syria, which was the suspected destination for the cargo.
The EU Syrian Sanctions Regime, established in May 2011, comprises financial, trade and transport penalties.
Gibraltar says the ship was detained when it "freely navigated" into their territorial waters.
Foreign Secretary Jeremy Hunt said the "bold move" by Gibraltar and the Royal Marines had denied valuable resources to Bashar Assad's "murderous regime".
The seizure comes amid heightened tensions between Iran and the US over Tehran's unravelling nuclear deal with world powers.