The British Foreign Office summoned Iran's ambassador for the second time in two days, saying an under-secretary had spent more than an hour in "frank and civil" talks demanding the safe return of the sailors and Royal Marines, and seeking assurances about their welfare and access to British consular officials.

Commodore Nick Lambert, commander of the Royal Navy frigate HMS Cornwall, speaks aboard his ship Friday March 23, 2007 in this image made from television. [AP]

Iran's top military official, Gen. Ali Reza Afshar, said the sailors and marines were moved to Tehran and under interrogation "confessed to illegal entry" and an "aggression into the Islamic Republic of Iran's waters." Afshar did not say what would happen to the sailors.

Iranian Foreign Ministry spokesman Mohammad Ali Hosseini accused the British of "violating the sovereign boundaries" of Iran, calling the entry a "blatant aggression."

He accused Britain of trying to cover up the incursion, saying it should "refrain from putting the blame on others."

The UN Security Council, meanwhile, unanimously voted to impose new sanctions against Iran for its refusal to stop enriching uranium -- a move intended to show Tehran that defiance over its nuclear program will leave it increasingly isolated.

British opposition lawmakers called on the government not to allow Iran to use the capture of the military personnel as a tool in the nuclear dispute.

"The United Kingdom will not be blackmailed. Iran has a choice: to act responsibly; or face greater isolation," said Menzies Campbell, leader of the opposition Liberal Democrats.

But the British government appeared to be avoiding harsh language in its public statements as it continued to gather information about exactly what had happened and why.

The British sailors had searched a merchant ship Friday morning when they and their two inflatable boats were intercepted by Iranian vessels near the disputed Shatt al-Arab waterway, US and British officials said. The Iranians surrounded them and escorted them away at gunpoint.

Britain immediately demanded the return of the eight Royal Navy sailors and seven Royal Marines -- at least one of who was a woman -- and denied they had strayed into Iranian waters while searching for smugglers off Iraq's coast.

Foreign Secretary Margaret Beckett and the Ministry of Defense said the troops were in Iraqi waters when they were seized.

Iraq's military commander of the country's territorial waters, Brig. Gen. Hakim Jassim, told AP Television News that Iraqi fishermen had reported that the British boats were "in an area that is out of Iraqi control."

In June 2004, six British marines and two sailors were captured, then paraded blindfolded on Iranian television. They admitted they had entered Iranian waters illegally but were released unharmed after three days.

Iranian hard-liners have already called for the 15 Britons to be held until Iran wins concessions from the West.

Several conservative student groups urged the Iranian government not to release the sailors until five Iranians detained by US forces in Iraq earlier this year are freed and UN's new sanctions against Iran are canceled. Some 500 Iranian students gathered on the shore near where the soldiers were captured, shouting "Death to Britain" and "Death to America," the semiofficial Fars news agency reported.

With tensions already running high, the United States has bolstered its naval forces in the Persian Gulf in a show of strength directed at Iran. There is concern that with so much military hardware in the Gulf, a small incident could escalate dangerously.

Afshar, the Iranian officer, warned the United States would not be able to control the consequences if it attacks Iran.

"The United States and its allies know that if they make any mistake in their calculations ... they will not be able to control the dimensions and limit the duration of a war," Afshar said.

Ayatollah Ali Khamenei, Iran's supreme leader, warned this week that Western countries "must know that the Iranian nation and authorities will use all their capacities to strike enemies that attack."

The Britons were seized in an area where the boundaries of Iraqi and Iranian waters have long been disputed. A 1975 treaty set the center of the Shatt al-Arab -- the 125-mile-long channel known in Iran as the Arvand River -- as the border.

But Saddam Hussein canceled that treaty five years later and invaded Iran, triggering an eight-year war. Virtually all of Iraq's oil is exported through an oil terminal near the mouth of the channel.

Iran and the new Iraqi government have not signed a new treaty on their sovereignty over the waterway.

The seized sailors, from the British frigate HMS Cornwall, are part of a task force that maintains security in Iraqi waters under authority of the UN Security Council. Cornwall's commander, Commodore Nick Lambert, said he hoped the detention was a "simple mistake" stemming from the unclear border.