The British troops - including one woman - were detained on Friday during what Britain described as a "routine anti-smuggling operation" in the Shatt al-Arab waterway that divides Iraq and Iran.
Iran's Fars news agency said on Saturday that the sailors and marines had been taken to Tehran to explain their "aggressive action".
General Ali Reza Afshar, a senior Iranian military official, told an Iranian news agency on Saturday that the captured Britons had "confessed to illegal entry into Iran's waters."
"The said personnel are being interrogated and have confessed to aggression into the Islamic Republic of Iran waters," Afshar was quoted as saying by the semi-official ISNA news agency.
In London Rasoul Movahedian, Iran's ambassador to the UK, was summoned for talks with Sir Peter Ricketts, a senior aide to Margaret Beckett, the British foreign secretary, which were described as "brisk but cordial".
Shortly afterwards, Beckett called on Tehran to fully explain the detentions, saying that Movahedian "was left in no doubt that we want them back".
British officials declined to confirm whether Movahedian had offered any assurances that the personnel would be released.
The Britons from the frigate HMS Cornwall, were in two inflatable boats and engaged in a "routine smuggling investigation" when they were seized, the US defence department said.
|The HMS Cornwall is one of the many British|
and US warships patrolling Iraqi waters [EPA]
The abduction of the British servicemen - including one woman - came as the UN Security Council prepared to vote on new sanctions aimed at forcing Tehran to curb its nuclear programme.
In June 2004, six British marines and two navy sailors were detained for three days in Iran after a routine operation.
They were paraded blindfold on television and forced to apologise for their "mistake".
On that occasion Iran insisted that the boats - which it has not yet returned - were intercepted only after they entered Iranian waters on the Shatt al-Arab that divides Iraq from Iran.
According to a statement from the US 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".
Commander Kevin Aandahl 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 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, Aandahl said.
A 1975 treaty gave the waters to Iraq and US and British ships commonly operate there, but Aandahl said Iran disputes Iraq's jurisdiction over the waters.