Iranian naval vessels seized 15 British sailors and marines Friday in Iraqi waters, the Ministry of Defense said.
Iran had no immediate comment about the incident. The British government summoned the Iranian ambassador, Rasoul Movahedian, to the Foreign Office for a meeting which a department spokesman described as "brisk but cordial."
During the meeting, Sir Peter Ricketts, the senior civil servant in the department, demanded "the safe return of our personnel and equipment," the spokesman said, speaking on condition of anonymity under department rules.
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Britain's Foreign Secretary Margaret Beckett demanded Teheran fully explain the detention, saying in a statement after Movahedian's summons that he "was left in no doubt that we want them back."
There was no comment from the Iranian side. A voice message at the embassy press office said it was closed until April 2.
The US Navy, which operates off the Iraqi coast along with British forces, said the British sailors appeared unharmed and that Iran's Revolutionary Guard naval forces were responsible.
The British sailors were assigned to a task force which protects Iraqi oil terminals and maintains security in Iraqi waters under authority of the UN Security Council.
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.
They were released unharmed after three days.
Britain, with the United States, France, Germany, Russia and China tried on Thursday to persuade all 15 nations on the UN Security Council to back new sanctions on Iran over its nuclear program.
The UN Security Council is set to vote on any sanctions resolution on Saturday.
