British Prime Minister Tony Blair has insisted no deal was done to free 15 Royal Navy crew members, who arrived safely at Heathrow on Thursday after being held in Iran for two weeks.

They were released "without any deal, without any negotiation, without any side agreement of any nature," he said.

"At the same time as we are open to bilateral dialogue and to pursue the lines of communication that have been opened up in the last two weeks, we have to hold absolutely firm in relation to support from any aspect of the Iranian regime for terrorism," he said.

Blair said he was glad for the return, but was grieving for the loss in Basra.

The British Ministry of Defence confirmed on Thursday that four soldiers had been killed in a roadside bomb blast and a fifth was seriously injured.

Blair said it was "far too early" to point to any Iranian involvement in that particular attack.

But he added: "The general picture, as I have said before, is there are elements at least of the Iranian regime that are backing, financing, arming, terrorism in Iraq."