LONDON (AP) - The next two days are “fairly critical” to resolving the dispute over a seized British navy crew, British Prime Minister Tony Blair said today, after Iran's chief international negotiator offered a new approach to end the standoff with Tehran.

Blair told Scotland's Real Radio that Ali Larijani's suggestion of talks offered hope of an end to the crisis. “If they want to resolve this in a diplomatic way the door is open,” the prime minister said.

But if negotiations to win the quick release of the 15 sailors and marines stalled, Britain would “take an increasingly tougher position,” he said.

The navy crew was detained March 23 by naval units of the Iranian Revolutionary Guards while the Britons patrolled for smugglers near the mouth of the Shatt al-Arab, a waterway that long has been a disputed dividing line between Iraq and Iran.

Iran says the team was in Iranian waters. Britain insists it was in Iraqi waters working under a U.N. mandate.

Iran has previously demanded an apology from Britain as a condition for the sailors' release.

Blair said today that Britain had two options in its approaches with Tehran.

“One is to try to settle this by way of peaceful and calm negotiation to get our people back as quickly as possible,” he said. “The other is to make it clear that if that is not possible that we have to take an increasingly tougher position.”

On Monday, Larijani said that Iran sought “to solve the problem through proper diplomatic channels” and proposed having a delegation determine whether British forces had strayed into Iranian territory in the Persian Gulf. He did not say what sort of delegation he had in mind.

Larijani told Britain's Channel 4 news Monday through an interpreter that Iranian officials “definitely believe that this issue can be resolved and there is no need for any trial.”

Earlier Monday, an Iranian state-run television station said all 15 of the detained Royal Navy personnel had confessed to illegally entering Iranian waters before they were captured.

However, Iranian state-run radio said the confessions would not be broadcast because of what it called “positive changes” in the negotiating stance of Britain, whose leaders have been angered by the airing of videos of the captives.

The radio did not elaborate on the supposed changes by the British. But in London, a British official, speaking on condition of anonymity because of the sensitivity of the issue, said Prime Minister Tony Blair's government had agreed to consider ways to avoid such situations in the future.

The official insisted Britain was not negotiating with the Iranians and still wanted the captives freed unconditionally.

Over the weekend, The Sunday Telegraph of London said Britain was considering sending a senior Royal Navy officer to Tehran to discuss the return of the captives as well as to talk about ways to avoid future incidents.

Larijani also urged Britain to guarantee “that such violation will not be repeated,” but avoided repeating Tehran's demand for an apology. British leaders have insisted they have nothing to apologize for.

The comments suggested the sides were seeking a face-facing formula in which each could argue its interests were upheld while the captives could go free. Under such a formula, Iran could claim Britain tacitly acknowledged the border area is in dispute, and Britain could maintain it never apologized.

A generation ago, such a formula helped free Americans held by Tehran for 444 days. The United States pledged not to interfere in Iranian affairs, enabling the hostage takers to claim they had achieved their goal.

The renewed diplomatic efforts between Iran and Britain followed tough rhetoric last week that prompted both governments to dig in their heels.

Britain suspended all other diplomatic contacts with Iran, froze work to support trade missions and stopped issuing visas to Iranian diplomats. It also sought help from the U.N. and other countries, including Muslim Turkey, to press Iran to free the captives.

Those moves prompted Iran to suspend plans to free the only woman captive, sailor Faye Turney, and to suggest the Britons might face trial.

To reinforce their claims, the Iranians also broadcast video footage that showed four of the crew saying they were captured in Iranian waters. In footage Sunday, two of the sailors used maps to show the purported location where they were seized.

Britain has released its own maps and GPS coordinates showing the captured team's location to be in Iraqi waters.

The videos enraged British officials, who said the broadcast confessions were clearly made under duress.

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Associated Press writers Nasser Karimi in Tehran, Iran, and Robert H. Reid in Amman, Jordan, contributed to this report.