British Prime Minister Tony Blair has accused elements in the Iranian regime of "financing, arming and supporting" terrorist attacks on UK forces in Iraq as four more British troops were killed in a roadside bomb attack.
As the 15 Royal Navy personnel freed after 13 days in Iranian captivity touched down at Heathrow Airport, Mr Blair said it was essential to remain "steadfast" in the face of Tehran's nuclear programme and continuing support for terrorism.
He contrasted the rejoicing at their return with the "sober and ugly reality" of events in Iraq, where British forces suffered their bloodiest day since November.
While Mr Blair acknowledged that it was premature to link Iran to the latest attack, he said it was clear that sources in the country had been involved in previous such incidents.
"The general picture, as I said before, is that there are elements - at least - of the Iranian regime that are backing, financing, arming, supporting terrorism in Iraq," he said.
British officials have previously accused Iran's Revolutionary Guards of supplying the sophisticated "shaped" projectiles used by Iraqi insurgents in a series of deadly roadside bomb attacks.
While Mr Blair said that the dialogue with Tehran sparked by the capture of the Navy personnel had opened up "new and interesting lines of communication", he said there was also a need to "reflect on our relationship with Iran".
In the latest incident in Iraq, four soldiers and a Kuwaiti interpreter were killed when their Warrior armoured vehicle hit a roadside bomb in the Hayaniyah district west of Basra in the early hours of the morning.
A fifth soldier was seriously injured in the blast, which left a crater a metre deep in the road.
A British military spokeswoman in Basra said they had been on a routine patrol to look for weapons and "anti-Iraqi force activity" but had made no finds and held no detainees.
However, Iraqi police officials claimed that the patrol had earlier detained a lieutenant in the Interior Ministry's major crimes unit, and were returning to their base when they were ambushed.
The officials said that after the bomb explosion, gunmen opened fire on the patrol causing more casualties. The killings brought the British death toll in Iraq this week to six.