Prime Minister Tony Blair on Sunday called the Iranian seizure of 15 British sailors and marines "unjustified and wrong," saying in his first remarks on the escalating confrontation that London saw it as a "very serious situation."
Iran said the issue was being "considered legally," suggesting the possibility that the group may be tried for illegally entering Iranian waters off the coast of Iraq.
U.S. Secretary of State Condoleezza Rice insisted during a trip to the Middle East that the Britons be released, saying "we all fully trust the British" account.
Blair disputed Tehran's claim that the 15 were in Iranian territorial waters at the time they were seized on Friday.
"There is no doubt at all that these people were taken from a boat in Iraqi waters," Blair said during a European Union meeting in Berlin.
"It is simply not true that they went into Iranian territorial waters, and I hope the Iranian government understands how fundamental an issue this is for us. We have certainly sent the message back to them very clearly indeed. They should not be under any doubt at all about how seriously we regard this act, which is unjustified and wrong."
Britain and the United States have said the sailors and marines were intercepted just after they completed a search of a civilian vessel in the Iraqi part of the Shatt al-Arab waterway, where the border with Iran has historically been disputed.
"I have not been commenting up to now because I want to get it resolved in as easy and diplomatic a way as possible," Blair said. "But this is a very serious situation."
In a telephone conversation with British Foreign Secretary Margaret Beckett late Sunday, Iranian Foreign Minister Manouchehr Mottaki gave no firm commitment on the service members' release, or even whether British officials would be allowed to meet them, the British Foreign Office said.
"The charge against them is illegal entrance into Iranian waters," Mottaki said in Persian through a translator at a news conference in New York on Sunday. "In terms of legal issues, it's under investigation."
Mottaki declined to provide the exact coordinates of where the Britons were seized, saying this "very detailed information has been submitted to the representatives of the United Kingdom."
A spokesman for Britain's defense ministry said London was not releasing the coordinates.
Beckett reiterated that the sailors and marines had been searching for smugglers in Iraqi waters under an agreement with the Baghdad government when they were seized by the naval forces of Iran's Revolutionary Guard, the Foreign Office said.
"The Iranian authorities intercepted these sailors and marines in Iranian waters and detained them in Iranian waters. This has happened in the past, as well," Mottaki said.
The Iranian state news agency IRNA said that Ibrahim Rahimpour, the foreign ministry official in charge of western Europe, had told British Ambassador Geoffrey Adams that the British sailors and marines were "well and sound" and that "legal proceedings" were under way.
Iran's top military official, Gen. Ali Reza Afshar, said on Saturday the seized Britons were taken to Tehran for questioning and had confessed to what he called an "aggression into the Islamic Republic of Iran's waters."
The capture and detention of the British service personnel increased tensions between Iran and the West already high over Tehran's nuclear program and allegations that Iran is interfering with the U.S.-led war in Iraq.
The U.N. Security Council agreed Saturday to tougher sanctions against Iran for its refusal to meet U.N. demands that it halt uranium enrichment. Many in the West fear the country's civilian nuclear research is cover for a weapons program, a claim Iran denies.
Iranian hard-liners have already called for the 15 Britons to be held until Iran wins concessions from the West.
British, Israeli and Saudi media reports on Sunday suggested that Iran was hoping to trade them for Iranian officials it claims have been abducted by the West in recent months.
Ali Askari, former head of an elite unit of the Revolutionary Guard, disappeared in Turkey six weeks ago; several months earlier, six Iranian officials were captured by U.S. forces an Iranian liaison office in Irbil, the capital of the Kurdish self-ruled region of Iraq. One was later released.
Iran said it was a government liaison office. The U.S. military said those detained were connected to an Iranian Revolutionary Guard unit that funds and arms insurgents in Iraq.
Sobh-e-Sadegh, the official publication of the Revolutionary Guards, said in a January article that it would be easy to kidnap Americans and transfer them to "any location of choice" in retaliation for any attack.
According to IRNA's English-language Web site, Adams said during the meeting in Tehran that the British service members had been deployed in Iraq to establish security, and had no hostile intention toward Iran.
"Tehran has always exercised self-restraint in the face of border violations by the British troops," Rahimpour was quoted as saying. But after the "contradictory statements" in the seizure of the British, the case "required an inquiry into such suspicious events."
Rajanews.com, a Persian Web site of supporters of Iranian President Mahmoud Ahmadinejad, quoted a senior diplomat as saying the Britons were still being held by Iranian armed forces and would not be released until they promised "not do similar things in future."
Ahmad Bakhshaysh, a political analyst and professor in politics in Tehran's Allameh University, said a prisoner swap was not what Iran wanted.
"Iran is not after retaliation regarding abduction of its diplomats. ... However, Iran will use this opportunity to show to the world public opinion that Britons were (the) invader and Iran was victim of the Westerners bullying policy," he said.
The capture of the British sailors and marines was not the first time Iranians have taken Western forces by surprise in the border area.
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.
U.S. News and World Report, citing a U.S. Army report out of Iraq, said American troops working with Iraqi border guards within Iraq were attacked by a much larger Iranian military unit in September. U.S. News said no Americans were hurt in the incident, but four Iraqi soldiers, an interpreter, and an Iraqi border policeman remain missing.
The U.S. military said the account was accurate, adding that the incident with the American troops, who were training, advising and helping the Iraqi border police, could have been a result of confusion in the vast desert area along the border.
"There is a lot of open terrain," military spokesman Lt. Col. Mike Donnelly said in an e-mail. "Visual sighting and happenstance encounters from a distance occur routinely."