Britain is joining an American military campaign to blunt Iranian influence in Iraq and the Gulf.
In a move likely to heighten tension in an already volatile part of the world, US forces have been ordered to detain Iranian agents in Iraq and to strengthen substantially Americas military presence in the Gulf.
Two Royal Navy minehunters have arrived in the Gulf to reinforce a naval frigate on patrol in the area.
We are going after their [Irans] networks in Iraq, Zalmay Khalilzad, the outgoing US Ambassador to Baghdad, said. The aim was to change the behaviour of the Islamic regime in Tehran, he added.
Robert Gates, the US Defence Secretary, accused Tehran of very negative behaviour.
Twice in the past few weeks US forces have detained Iranian officials in Iraq, first in Baghdad and last week in the northern city of Arbil.