British officials expressed their deep concern regarding the incident with the arrest of 15 British sailors and marines in the waters of Iraq. The British government asked for immediate explanations from the Iranian ambassador. The officials demanded "the immediate and safe return of our people and equipment."
Iran had no immediate comment about the incident. In London, the British government summoned the Iranian ambassador and demanded "the immediate and safe return of our people and equipment."
The U.S. 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.
Britain's Defense Ministry said the British Navy personnel were "engaged in routine boarding operations of merchant shipping in Iraqi territorial waters," and had completed a ship inspection when they were accosted by the Iranian vessels.
"We are urgently pursuing this matter with the Iranian authorities at the highest level," the ministry said.
No one could be immediately reached for comment at either government offices in Iran or at the Iranian Embassy in Baghdad or at the U.N. mission.
Iran is in the middle of its New Year holiday when almost all government offices close.
A U.S. official said the incident occurred just outside a long-disputed waterway called the Shatt al-Arab dividing Iraq and Iran. It came as tensions were running high in the Persian Gulf after Iran's defiance of U.N. Security Council orders to rollback on its nuclear program and U.S. allegations that Iran is arming Shiite militias in Iraq.
U.S. officials had expressed concern that with so much military hardware concentrated in the Persian Gulf, just such a small incident could spiral out of control and trigger a major armed confrontation.
White House press secretary Tony Snow said the Bush administration was monitoring the situation.
"The British government is demanding the immediate safe return of the people and equipment and we are keeping watch on the situation," Snow said.
The United States, Britain's chief ally, has built up its naval forces in the Gulf in a show of strength directed at Iran. Two American carriers, including the USS John C. Stennis - backed by a strike group with more than 6,500 sailors and Marines and with additional minesweeping ships - arrived in the region in recent months, ratcheting up tensions with Iran.
Earlier this week, Ayatollah Ali Khamenei said if Western countries "want to treat us with threats and enforcement of coercion and violence, undoubtedly they must know that the Iranian nation and authorities will use all their capacities to strike enemies that attack."
In February, President Bush said: "The Iranian people are good, honest, decent people and they've got a government that is belligerent, loud, noisy, threatening - a government which is in defiance of the rest of the world and says, 'We want a nuclear weapon."'
The Britons were in two boats from the frigate H.M.S. Cornwall during a routine smuggling investigation, said the British Defense Ministry.
According to a statement from the U.S. Navy's Fifth Fleet, which is based in Bahrain and operates jointly with the British forces off the coast of Iraq, the British sailors had just finished inspecting the merchant ship "when they and their two boats were surrounded and escorted by Iranian vessels into Iranian territorial waters."
Cmdr. Kevin Aandahl of the Fifth Fleet said the British crew members were intercepted by several larger patrol boats operated by Iranian sailors belonging to the Iranian Revolutionary Guard Corps Navy, a radical force that operates separately from the country's regular navy.
The Iranian boats normally carry bow-mounted machine guns, while the British boarding party carried only sidearms, Aandahl said. No shots were fired and there appeared to be no physical harm done to any personnel involved or their vessels, Aandahl said.
The seizure of the British vessels, a pair of rigid inflatable boats known as RIBs, took place in long-disputed waters just outside of the mouth of the Shatt al-Arab waterway that divides Iraq from Iran, Aandahl said. A 1975 treaty gave the waters to Iraq and U.S. and British ships commonly operate there, but Aandahl said Iran disputes Iraq's jurisdiction over the waters.
"It's been in dispute for some time," Aandahl said. "We've been operating there for a couple of years and we know the lines very well. This was a compliant boarding, this happens routinely. What's out of the ordinary is the Iranian response."
Aandahl said the U.S.-led task force has touchier relations with the Revolutionary Guard, who often ignores normal maritime operating traditions, than with the regular Iranian navy.
The Shatt al-Arab waterway is formed by the confluence of the Tigris and Euphrates rivers at the town of Al-Qurnah. It flows southeastward for 120 miles (193 km) and passes the port of Basra and the Iranian port of Abadan before emptying into the Persian Gulf.
A fisherman who said he was with a group of Iraqis from the southern city of Basra fishing in Iraqi waters in the northern area of the Gulf said he saw the Iranian seizure. 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."
The Cornwall's commander, Commodore Nick Lambert, said the frigate lost communication with the boarding party, but a helicopter crew saw the Iranian vessels approach.
"I've got 15 sailors and marines who have been arrested by the Iranians and my immediate concern is their safety," Lambert told British Broadcasting Corp. television, reports AP.
Lambert said it was a routine boarding, the skipper of the vessel "answered all the questions, and the leader of the boarding party cleared him to continue with his business."
In June 2004, six British marines and two sailors were seized by Iran in the Shatt al-Arab. They were presented blindfolded on Iranian television and admitted entering Iranian waters illegally, then released unharmed after three days.