Iran may delay its plan to release the only woman among 15 captured Royal Navy crew because of the UK's "incorrect attitude", an official has said.
Ali Larijani, Iran's chief nuclear negotiator, appeared to be referring to the UK's refusal to agree the crew had been in Iranian waters.
Iran's insistence the group was in its waters last Friday saw the UK release GPS evidence to back up its denials.
Tony Blair's official spokesman said the UK was not seeking a confrontation.
Mr Larijani, head of Iran's supreme national security council, said: "It was announced that a woman in the group would be freed, but (this development) was met with an incorrect attitude."
He later added on Iranian state radio the UK had "miscalculated" in its handling of the case.
"They have created a ballyhoo over diplomatic ties and raised the issue with the UN," he said.
"This will not help solve the issue. The British leaders are miscalculating the case."
UN backing sought
BBC diplomatic correspondent James Robbins said Mr Larijani appeared to be responding to the failure to secure an admission from the UK.
Mr Blair's spokesman said footage of Leading Seaman Faye Turney and the others on Iranian TV was wrong in terms of "basic humanity".
The Foreign Office said the UN was preparing to issue a statement calling for the release of the crew.
UN Secretary-General Ban Ki-moon held talks with Iran on the detention at a summit in Riyadh.
And the European Union foreign policy chief Javier Solana said he had spoken briefly to Iran's foreign minister and reiterated the EU's call for an end to the "illegal" detention.
The prime minister's spokesman said: "What is important is that Iran knows that it is isolated on this issue.
UK VERSION OF EVENTS
1 Crew boards merchant ship 1.7NM inside Iraqi waters
2 HMS Cornwall was south-east of this, and inside Iraqi waters
3 Iran tells UK that merchant ship was at a different point, still within Iraqi waters
4 After UK points this out, Iran provides alternative position, now within Iranian waters
"We are not seeking to put Iran in a corner. We are simply saying, 'please release the personnel who should not have been seized in the first place'."
He said a pledge by Iranian foreign minister Manouchehr Mottaki that British officials in Tehran would be given consular access to the personnel had not taken place.
On Wednesday, Iranian state television broadcast an interview with LS Turney and footage of the 14 servicemen seized with her.
The spokesman said: "Nobody should be put in that position. It's an impossible position. It's wrong in terms of the UN conventions which govern this, and it's wrong in terms of basic humanity."
Iran's foreign minister had previously said LS Turney could be released.
But he said the stand-off would be resolved only if the UK stated that the sailors and marines had been in Iranian waters last Friday, which it denies.
Iranian state-run television reported LS Turney would not be released "for the time being".
Those who know the sailor have spoken of their shock at the news her release may be delayed.
The Rev Sue Caddy, vicar at Oxon Church, near Shrewsbury, where LS Turney was married in 2002, said the congregation was praying for her.
Nicola Jones, who lives near LS Turney's home in Plymouth, said: "It's really shocking.
"She's a nice girl - friendly, happy and chirpy. It's really quite sad this has happened."
Iran has not disclosed where the service personnel are being held but the BBC has been told they are at a military base in Tehran.
Deputy Chief of the Defence Staff, Vice Admiral Charles Style, has produced data which he said proved the boarding party was 1.7 nautical miles inside Iraqi waters.
The Iranian embassy in London said the Britons had been 0.5 km inside Iranian territorial waters.