The US Defence Secretary suggested for the first time last night that American forces could be in Iraq for at least another half century, under an arrangement similar to the effectively permanent US troop deployment in South Korea.

In comments that will dismay war opponents at home and alarm Muslim allies in the Middle East, Robert Gates said that “some force of Americans” will be in Iraq for a “protracted period of time”, and pointed to South Korea as the model.

US troops have been in South Korea since the end of the 1950-53 Korean War, in the heavily armed demilitarised zone that separates the country from North Korea. US generals are in charge of the combined US-South Korean forces.

Mr Gates, speaking to reporters in Hawaii during a visit to US Pacific Command, said that current war plans still called for an assessment of the US “surge” strategy in September but he was looking beyond that to the type of military presence the US will have in Iraq over the long term.

He contrasted the situation in South Korea to Vietnam, where, he said, “we just left, lock, stock and barrel”, a reference to the US withdrawal after the fall of Saigon in 1975.

“What I’m thinking in terms of is a mutual agreement where some force of Americans – mutually agreed with mutually agreed missions – is present for a protracted period of time,” he said. “The idea is more a model of a mutually agreed arrangement whereby we have a long and enduring presence but under the consent of both parties and under certain conditions. The Korea model is one, the security relationship we have with Japan is another.”

All eyes in Washington are on the progress report to Congress in September by General David Petraeus, the US ground commander, with moderate Republicans saying that anything less than significant optimism will end their support for President Bush.

Yesterday General Raymond Odierno, the No 2 commander in Iraq, told reporters in Washington via video link from Baghdad that he may not be able to make a full assessment by September of whether the build-up is succeeding in stabilising Iraq.

The Bush Administration said yesterday that it was at last ready to admit up to 7,000 Iraqi refugees into the US, after months of delays and growing criticism of its refusal to grant asylum to some of the 2.2 million Iraqis who have fled the war. Refugee groups welcomed the move but condemned it as too little, too late. The US has admitted fewer than 800 Iraqi refugees.

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