Marines from 539 Assault Squadron Royal Marines conduct operations on waterways in the Basra region of southern Iraq as part of Operation Troy in this recent but undated image released by the British Royal Marines. Iranian naval vessels on Friday seized 15 British sailors who had boarded a merchant ship in the Persian Gulf.

London ? 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 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.”