Britain is still waiting to be granted access to 15 Royal Navy personnel held by Iran, two days after their capture.
The 15 are based on HMS Cornwall, which patrols Iraqi territorial waters
In a meeting in Tehran between Iranian government officials and the UK ambassador, Iran also failed to say where the 15 were being held.
Minister Peter Hain said the seizure of the eight sailors and seven marines, from HMS Cornwall, was "very serious".
Iran says the Britons, seized on Friday, were trespassing in Iranian waters, which the UK denies.
The Foreign Office is adamant they were in Iraqi waters and has called for their immediate release.
The British embassy said it had sought Sunday's meeting to demand the release of the sailors.
However, reports on Iranian TV said it was Iran's foreign ministry that summoned the ambassador, to receive a protest about British actions.
The Britons, who include one woman, were seized at gunpoint by forces said to be Iranian Revolutionary Guards, after inspecting an Iraqi boat and returning to their two small boats to head back to the Cornwall.
British ambassador Geoffrey Adams asked to know the whereabouts of the personnel and for consular access, but this was not forthcoming.
A British diplomat said the Iranians replied they would get back to the embassy on those issues - and told the ambassador the release of the 15 Britons was yet to be determined.
BBC News security correspondent Gordon Correra said Iran was playing a "tough game" over the situation.
Foreign Office Minister Lord Triesman met the Iranian ambassador in London on Saturday to demand the group's immediate release.
But on Sunday he told Sky News: "We do not know where they are, I wish we did."
He said the Royal Navy commander in the area was confident they had the technical evidence to prove the personnel were not in Iranian waters.
Efforts to get more information may be being hampered by the two-week new year public holiday in Iran.
Northern Ireland Secretary Mr Hain said: "We're pursuing every negotiating option to try and bring our soldiers back and make sure that they're safe.
"It's essential that this occurs and it's essential not just for the well-being of our soldiers but also for stability in the region."
Students belonging to the paramilitary Basij group, which is close to Iranian President Mahmoud Ahmadinejad, have called for the Britons to be put on trial.
However, Professor Sadaq Ziba-Kalam, of Tehran University, told BBC News 24 he did not think they would be charged with spying.
"That part of the water between Iran and Iraq where the incident happened has been disputed for decades," he said.
"So it is very difficult to draw the line and say this is the Iranian side of the border and this is the Iraqi side of the border."
The Cornwall is the flagship of the coalition-Iraqi force which patrols Iraqi territorial waters in the northern Gulf to combat smuggling.
Iranian armed forces spokesman Gen Ali Reza Afshar told Iranian media on Saturday that the 15 personnel were being interrogated, but were in good health.
They had confessed to being in Iranian waters, he said. But ex-Navy chief Admiral Sir Alan West said a confession in such circumstances meant "absolutely nothing".
Germany - which holds the EU presidency - called for the immediate release of the Britons.
On Saturday the UN Security Council voted unanimously in favour of further sanctions against Iran for its refusal to suspend its nuclear enrichment programme.
The seizure of the boarding party carries echoes of an incident in June 2004 when a group of eight marines and sailors were held for three days after being seized by the Iranians in the Shatt al-Arab waterway.
They were paraded blindfolded on television, and later freed.