Margaret Beckett last night held out little hope of a quick resolution to the diplomatic crisis with Iran but much could rest on an eagerly awaited statement expected today from Mahmoud Ahmadinejad, the Iranian President.

Some 12 days into the stand-off - covering the prolonged New Year public holiday in Iran - it appears there has still been no formal contact with the leadership in Tehran.

Tony Blair, while stressing how the "door is open" on a diplomatic solution, made clear the next 48 hours would be "critical" in the government's attempt to free the 15 British sailors, who have been held captive for nearly two weeks.

He warned Britain would be prepared to adopt "increasingly tougher decisions", if there was no progress towards releasing the captives.

The Foreign Secretary made clear this referred to diplomatic efforts, not military action. "We are not seeking confrontation but to pursue this through diplomatic channels," she stressed.

Following another daily meeting of Cobra, the government's emergency committee, Ms Beckett told journalists at a briefing in Whitehall: "I would urge you to be cautious in assuming that we are likely to see a swift resolution to this issue." She stressed that UK diplomatic efforts would continue "intensively".

She said there was "a huge amount going on behind the scenes". David Triesman, the junior Foreign Office Minister, for example, has had seven meetings with the Iranian ambassador in London in 11 days, and last night was due to have his eighth. Similar meetings are taking place in Tehran. Yet, it appears there has yet to be any formal top-level exchange.

The Secretary of State confirmed there had still been no consular access to the sailors nor had there been a formal response to an earlier UK note to the Iranian government.

She referred to Monday's television interview by Ali Larijani, the head of Iran's Supreme National Security Council, saying: "It's clear there remains some differences between us but we certainly share the preference he seemed to be expressing for bilateral discussions to move forward to a diplomatic solution to that problem. We are following up those remarks with the Iranian authorities."

The Foreign Secretary's remarks followed fresh pictures of the naval personnel released by an Iranian news agency in an apparent breach of the previous understanding that no more photographs would be published.

The pictures showed six sailors, smiling and relaxed, sitting in blue and red tracksuits on a carpet in a room, some playing chess.

A caption read: "British sailors are chatting and eating fruit, drinking coffee and playing chess. It seems the sailors are satisfied with their situation, in which they are enjoying good conditions instead of working in a hard situation in the Persian Gulf."

In Glasgow, on the Holyrood campaign trail, Mr Blair said: "The next 48 hours will be fairly critical," warning Tehran that Britain would take progressively tougher measures if there were no moves to free the captives.

He said the Monday interview, in which Mr Larijani said he wanted the dispute settled diplomatically, offered "some prospect" of a resolution.

The PM explained the government's twin-track approach: on one hand, making sure Iran "understands the pressure is there available to us if this thing has to be hard and tough and long" and on the other hand, making clear "we're not looking for confrontation over this and if they want to resolve this in a diplomatic way, the door is open".

Later, Mr Blair added: "This is a very critical time and the most important thing is we get our people back safe and sound, that is what we're working for.

"The next few days are going to be vital. It's probably not sensible for me to say any more than that; the most important thing is their welfare."

In a separate development, officials in London and Tehran declined to say whether the release of an Iranian diplomat in Iraq was linked to the issue of the captive sailors.

Jalal Sharafi, a secretary at the Iranian embassy in Baghdad, was seized two months ago by uniformed gunmen and was released unexpectedly yesterday; he was back in the Iranian capital within hours.

His release and Tehran's efforts to free five other Iranians seized in Iraq earlier this year have prompted speculation that a deal might be taking shape.

However, in all their statements so far, Iranian officials have insisted the capture of the British sailors is not linked to anything else.

In Washington, George W Bush rejected any talk of doing covert deals with Tehran.

The US President said: "I also strongly support the Prime Minister's declaration that there should be no quid pro quos when it comes to the hostages."