London, March 27 (DPA) British Prime Minister Tony Blair warned Tuesday that efforts to secure the 15 Royal Navy personnel captured by Iran would enter a "different phase" if the sailors and Marines were not released soon.

The British government's main concern was the welfare of the 14 men and one woman who should be released as "quickly as possible", Blair told BBC Tuesday.

The BBC said it had been told that the service personnel, seized last Friday in the northern Gulf, were being held at an Iranian Revolutionary Guards Corps base in Tehran.

"What we are trying to do at the moment is to pursue this through the diplomatic channels and make the Iranian government understand these people have to be released and that there is absolutely no justification whatever for holding them," said Blair.

"I hope we manage to get them to realise they have to release them. If not, then this will move into a different phase."

Asked what he meant by a "different phase," Blair said: "Well, we will just have to see, but what they should understand is that we cannot have a situation where our servicemen and women are seized when actually they are in Iraqi waters under a UN mandate, patrolling perfectly rightly and in accordance with that mandate, and then effectively captured and taken to Iran."

The most important thing is their welfare, I am trying to get this resolved in as diplomatic and sensible a way as possible," he added.

Iran has said that the eight sailors and seven Royal Marines, all crew members of the frigate HMS Cornwall, had illegally entered Iranian waters, but the British government insists that they were in Iraqi waters when seized following the inspection of a vessel.