Story Highlights • NEW: Iran claims British sailors and marines admit to being in Iranian waters

• Iran claims UK sailors entered Iranian waters in "suspicious act"

• 15 British personnel on patrol in the Persian Gulf "seized" by Iranian navy

• Incident could exacerbate tensions with West over Iran's nuclear program

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TEHRAN, Iran (CNN) -- Iran says the 15 British sailors and marines seized by Iran in the Persian Gulf have confessed to trespassing into Iranian territory, the semi-official FARS News Agency reported Saturday.

But the British Ministry of Defense would not confirm the report. (Watch reports on UK troops 'confessing' to being in Iran waters )

The British government said its request to communicate with the detained troops has not been granted by Iran.

Earlier Saturday, FARS reported that the sailors and marines -- including at least one woman -- were brought to Tehran to explain their "aggressive behavior." (Watch how British sailors and marines were seized )

The British Ministry of Defense said Friday the Iranian Revolutionary Guard Corps Navy surrounded and seized the marines while they were conducting a routine inspection on a merchant vessel. They were then "escorted by Iranian vessels into Iranian territorial waters." (Location of incident)

Iran's foreign ministry spokesman on Saturday condemned the British government for attempting to cover up its "blunder" and said it should stop "putting blame on others," according to IRNA, the state-run Iranian news agency.

Hours after reports of the arrests surfaced, a flurry of diplomatic activity ensued.

Britain announced it had called Iran's ambassador for a meeting and demanded the immediate release of the marines, while Iran announced on state-run TV that it had asked Britain's representative to Tehran to explain why the personnel had crossed into Iranian territory.

Iran waited until late Friday to release a statement. The report was aired on state-run TV and sourced Iranian foreign ministry officials. There was no mention of where the marines were being held or what would happen next, only that they were arrested for trespassing.

Foreign Secretary Margaret Beckett said Friday her office was making clear it expected the personnel to be released immediately, along with "a full explanation of what happened."

The European Union also called for the "immediate liberation" of the captured sailors.

A U.S. military official who monitors the region told CNN the marines stopped a vessel suspected of smuggling automobiles, and boarded it for an inspection. While the marines were on board, as many as six ships from the Iranian Revolutionary Guard Corps Navy showed up and claimed the British had entered Iranian waters.

A dispute ensued over whether the marines were on Iraqi or Iranian territorial waters, and the 15 were then seized along with the two small rigid hull inflatable boats they had used to get from their ship to the vessel, the U.S. military official said. The official insisted on anonymity because the incident did not involve the U.S. military.

Nick Lambert, commodore of the ship, said his marines were inside Iraqi territorial waters when they were arrested. There was no fighting and no use of weapons, he told a BBC pool reporter on the ship. "It was entirely peaceful."

The incident threatened to exacerbate the tension between Iran and much of the West on the eve of a U.N. Security Council vote to impose new sanctions on Iran. The world powers will meet Saturday to consider that next step in the dispute over Tehran's nuclear ambitions. (Full story)

There have been similar incidents in the past. In 2004, Iran stopped three British boats and seized eight sailors and six marines. The Iranian Foreign Ministry said at the time the three boats had crossed into Iran's territorial waters.

The detained service members appeared on Iranian television blindfolded.

They were released after Iran said it determined they had mistakenly crossed into Iran's waters.

CNN's Nicola Goulding contributed to this report