Iran's state-run television channel last night aired more videos of two captured British Marines.

The footage showed the pair pointing to a map and Al-Alam TV said they were identifying where their boat crossed into Iranian waters on March 23.

The Marines, who appeared in separate video clips wearing military fatigues, were talking to a camera, but Al-Alam did not air their voices.

Instead, the broadcaster said the two "have confessed" to trespassing into Iranian waters "illegally". The TV station also gave more details about the incident, saying the 15 left their ship in a small boat in the morning of March 23 and entered the Iranian waters at 10am local time. According to Al-Alam TV, the sailors are receiving "good and humanitarian treatment".

Earlier it was revealed that UK officials are in "direct bilateral communication" with their counterparts in Iran. The link emerged as the British Embassy in Tehran was pelted with rocks and firecrackers during a demonstration by around 200 students.

Calling for the British ambassador to be expelled from Iran and describing the embassy as a "den of spies", they chanted "death to Britain" and "death to America" as stones were hurled into the embassy's courtyard.

After days of heated rhetoric, the government was last night pinning its hopes on "unglamorous but important quiet diplomacy" to free the sailors.

Defence Secretary Des Browne said: "We're anxious this matter be resolved as quickly as possible and that it be resolved by diplomatic means. We're bending every single effort to that."

He added that the Iranians knew "that not only have we a very clear position but that we have the support of almost the whole of the international community; the message from the UN, the message from the EU ought to make it clear to them that their responsibility is to release our detained personnel".

Earlier, the Foreign Office declined to comment on reports that ministers were preparing a compromise face-saving deal in which a senior Royal Navy officer would act as a special envoy to Tehran, with an assurance that British forces would never knowingly enter Iranian waters without permission.

A departmental spokesman told The Herald: "We don't comment on leaks and we will not be commenting on speculation." However, the possibility of a special envoy carrying such a message was flagged up after a private note was passed to diplomats at Britain's embassy in Tehran over the weekend. This has led to a clear quietening down in the rhetorical exchanges.

The wording of any UK offer will, of course, be crucial as Tony Blair and the UK's naval commanders have repeatedly made clear that the 15 sailors who were seized while carrying out a routine smuggling investigation under a UN mandate were in Iraqi not Iranian waters.

No 10 was keen to point out how Douglas Alexander, Scottish Secretary, had explained the government's current strategy when he said yesterday: "Across government we have been very careful in saying there should be a graduated step by step response to this."

Mr Alexander went on: "The responsible way forward is to continue the often unglamorous but important quiet diplomatic work."

Mr Alexander, who is also the Transport Secretary, refused to be drawn on the prospect of a pan-European ban on flights in and out of Iran. He insisted the government was adopting a "twin track approach".

He added: "On the one hand, working closely with international partners to make clear the strength of international feeling that these British service personnel should be returned. And on the other hand, exploring the potential for dialogue with the Iranians."

Earlier, Mahmoud Ahmadinejad, Iranian President, called world powers "arrogant" for refusing to apologise.

He was speaking out after President George W Bush described the sailors as "hostages" and denounced Tehran for its "inexcusable behaviour".