LONDON: Britain said on Friday it was sending a second warship to the Gulf and raising the alert level in the region as tensions spike after Iranian gunboats threatened a UK supertanker.

The decision was disclosed as US President Donald Trump stepped up his war of words with the Islamic republic over its nuclear programme and its alleged support for terror groups in the Middle East.

`Iran better be careful,` Trump told reporters outside the White House.

`They`re treading on very dangerous territory. Iran, if you`re listening, you better be careful.` British officials said the naval deployment was part of a preplanned rotation and meant to ensure a continued British naval presence in one of the most important but volatile oil shipment routes in the world.

But a source said the switchover wasbrought forward by several days and should see two of Britain`s most advanced warships jointly navigating the waters off Iran for a period that might last a number of weeks.

A government spokesman said the HMS Duncan was `deploying to the region to ensure we maintain a continuous maritime security presence while HMS Montrose comes off task for preplanned maintenance and crew changeover` Sky News television reported that HMS Duncan would not be part of the international military escort team for vessels in the Gulf proposed on Thursday by the United States.

HMS Duncan is an air defence destroyer that carries a set of heavy H arpoon anti-ship missiles and a crew in excess of 280.

The BBCreported thatithadreachedTurkey`s Bosphorus Strait as it makes its way from the Black Sea through the Suez Canal to the Gulf.

HMS Montrose was forced to warn of f three fast Iranian gunboats on Wednesday that UK officials said were trying to `impede` the progress of a British supertanker through the Strait of Hormuz in the Gulf. Iranian officials have denied the incident ever happened.

The Strait of Hormuz episode occurred a week after UK Royal Marines helped the Gibraltar authorities detain an Iranian tanker that US officials believe was trying to deliver oil to Syria in violation of separate sets of EU and US sanctions.

Iran has bristled at the arrest and issued a series of increasingly ominous warnings to both the United States and Britain about its right to take unspecified actions in reprisal.-AFP