An October 2005 handout photo released in London on Thursday shows the British Royal Navy's HMS Montrose, a Type 23 frigate, performing turns during exercise "Marstrike 05," off the coast of Oman. Britain said on Thursday that Iranian military vessels tried to "impede the passage" of UK oil tanker British Heritage, but were warned off by British warship HMS Montose in a dramatic escalation of tensions with Tehran in the Gulf. Photo: AFP