LONDON, July 12 (Xinhua) -- Britain will send a second warship to the Gulf amid rising tensions with Iran, Sky News reported on Friday.

HMS Duncan, a Type 45 Destroyer, will sail to the gulf in the coming days and operate alongside the Royal Navy frigate HMS Montrose and U.S. allies there, although it will not be part of an international maritime coalition proposed by the U.S., said the report.

The police of Gibraltar, Britain's overseas territory, arrested on Thursday the captain and chief officer of an Iranian oil supertanker Grace 1, which was reportedly transporting Iranian crude oil to Syria.

British Marines and Gibraltar police intercepted the oil tanker on July 4 in the Strait of Gibraltar in the south of the Iberian Peninsula over alleged violation of the European Union's sanctions against Syria.

A spokesperson of Gibraltar police said the two men arrested were being investigated but have not been charged yet. Documents and electronic devices were also seized from the tanker.

Iranian President Hassan Rouhani on Wednesday warned Britain of the "consequences" following the seizure of an Iranian oil tanker.

"I remind the Britons: you are the ones initiating insecurity, and you will come to realize its consequences in the future," Rouhani said in a cabinet meeting, quoted by Press TV.