The 15 Britons held by Iran accept they were picked up in its waters despite the UK's insistence they were in Iraqi territory, Iranian state radio says.
It reported film of all the Royal Navy personnel giving details existed but would not be broadcast on TV because of "positive changes" in the UK's stance.
Later, previously unseen images of some of the crew were aired but their voices could not be heard.
The prime minister's spokesman said Iran needs to show "where it stands".
According to Iran's ISNA news agency further footage of all the crew "explaining details about their arrest in Iranian waters" would not be aired because of "changes seen in the last two days in the clamorous British government policies".
The film shown later on state-run Al-Alam satellite television cuts quickly between several of the men and includes footage of a group of three by a map.
The prime minister's spokesman said there was "a lot going on behind the scenes".
"What happens next depends on the Iranian response," he added.
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
The Foreign Office said it stood by its view the crew had been detained against their will in Iraqi territory.
"We have not had consular access, we have made it quite clear they were seized in Iraqi waters, and we want them released," a spokesman said.
At the weekend, Foreign Secretary Margaret Beckett said a diplomatic note had been sent to the Iranians.
Defence Secretary Des Browne has confirmed the government was in "bilateral" communication with Iran over the personnel seized in the northern Gulf on 23 March.
It is understood that one area under discussion is how future disputes over the contested waters may be avoided.
BBC world affairs correspondent Emily Buchanan said the UK's "tough stance" had seen it take the issue to the United Nations but there were indications diplomats had become more wary of upsetting negotiations.
Four crew members have so far been seen on the Al-Alam satellite television channel saying they had entered Iran's waters.
IRANIAN VERSION OF EVENTS
1 Royal Navy crew stray 0.5km inside Iranian waters
2 Iran gives set of co-ordinates to back up their claims
3 According to seized GPS equipment, the Royal Navy crew had previously entered Iranian waters at several other points
4 Iran informs Britain of the position where the crew were seized, inside Iranian waters
The Foreign Office has said showing the group on TV is "unacceptable" and it had "grave concerns" over the circumstances under which any statements were made.
The first footage showed Leading Seaman Faye Turney and Nathan Summers speaking.
On Sunday, Royal Marine Capt Chris Air and Lt Felix Carman were seen explaining the incident.
The seven British sailors and eight marines, based on HMS Cornwall, were seized by Revolutionary Guards as they returned from searching a vessel.
On Sunday, Iranian students threw stones and firecrackers at the British embassy in the capital Tehran in protest against what they said was the "illegal entry" into their waters by the UK personnel.