Britain says three Iranian boats tried to intercept a British oil tanker near the Persian Gulf in the Strait of Hormuz. London says they were driven off by a Royal Navy frigate - the HMS Montrose - escorting the tanker in the Strait of Hormuz. The frigate aimed its deck guns at the Iranian boats as a warning. No shots were fired. Iran has denied that its vessels tried to stop the tanker, saying they were carrying out routine duties. The incident comes after a warning yesterday from Iranian President Hassan Rouhani. He said that Iran was ready to retaliate after one of its oil tankers was seized off the coast of Gibraltar last week. The UK claims the Iranian tanker was delivering oil to Syria in violation of European Union sanctions.The British government called for calm after the incident.