President Bush warned Iran yesterday that if it did not stop arming and training Shia militia in Iraq then “America will act to protect our interests and our troops”.

During a speech in which he ordered an indefinite halt to US troop withdrawals from Iraq this summer, Mr Bush called Iran one of the two greatest threats to America in this century, together with al-Qaeda. His words echoed testimony from General David Petraeus, the US commander in Iraq, to Congress this week in which he described Iranian-backed Shia groups and the “malevolent” influence of Tehran as the biggest long-term threat to a viable Iraq.

Mr Bush said that he was following General Petraeus’s advice to halt withdrawals after July, when troop levels have been reduced to 140,000, roughly the same number deployed in Iraq before last year’s surge of additional troops. He said that General Petraeus — who called the situation in Iraq too fragile for further withdrawals — would have “all the time he needs” to evaluate further pullouts.

The decision means that Mr Bush’s successor will inherit Iraq with up to 140,000 troops still deployed.

Mr Bush spoke two hours after Ryan Crocker, the US Ambassador to Iraq, said that Washington was ready for talks with Iran. “We are prepared to do that,” he said. “If the Iranians are ready to sit we are going to point out what they are doing to undermine Iraqi security.”

Mr Bush announced that he would seek to relieve the heavy strain on the army by reducing the length of combat tours in Iraq and Afghanistan to 12 months, down from the current level of 15 months. The change would take effect on August 1, and would affect US forces already deployed on the front lines.

Colin Powell, Mr Bush’s former Secretary of State and chairman of the Joint Chiefs of Staff in the Gulf War, gave warning that the US military was “very, very stretched” by its commitments in Iraq and Afghanistan.” The burden would be “extremely difficult” for the US to sustain, he said.

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