Iranian naval vessels seized 15 British sailors and marines Friday in Iraqi waters, the Ministry of Defense said.
The British personnel from the frigate HMS Cornwall were "engaged in routine boarding operations of merchant shipping in Iraqi territorial waters," and had completed their inspection of a merchant ship when they were accosted by Iranian vessels, the ministry said in a statement.
"We are urgently pursuing this matter with the Iranian authorities at the highest level and ... the Iranian ambassador has been summoned to the Foreign Office," the ministry said.
"The British government is demanding the immediate and safe return of our people and equipment."
A fisherman who said he was with a group of Iraqis from Basra in the northern area of the Gulf said he witnessed the event. The fisherman declined to be identified because of security concerns.
"Two boats, each with a crew of six to eight multinational forces, were searching Iraqi and Iranian boats Friday morning in Ras al-Beesha area in the northern entrance of the Arab Gulf, but big Iranian boats came and took the two boats with their crews to the Iranian waters," said the fisherman.
The British Broadcasting Corp. said the British forces were inspecting a ship suspected of smuggling cars. It did not cite a source for the report.
BBC reporter Ian Pannell on HMS Cornwall said the sailors had just boarded a dhow when they were accosted.
"While they were on board, a number of Iranian boats approached the waters in which they were operating - the Royal Navy are insistent that they were operating in Iraqi waters and not Iranian waters - and essentially captured the Royal Navy and Royal Marine personnel at gunpoint," Pannell said.
In June 2004, six British marines and two sailors were seized by Iran in the Shatt al-Arab between Iran and Iraq. They were presented blindfolded on Iranian television and admitted entering Iranian waters illegally, reports AP.
They were released unharmed after three days.