About 275 US military personnel are being deployed to Iraq to help American personnel and protect the embassy in Baghdad, President Barack Obama has said in a letter to Congressional leaders.
The force, which began deploying on Sunday, has been sent "for the purpose of protecting US citizens and property, if necessary, and is equipped for combat," Obama wrote.
"This force will remain in Iraq until the security situation becomes such that it is no longer needed."
The move comes as jihadists of the Islamic State of Iraq and the Levant (ISIL) battle Iraqi security forces for control of a strategic northern town and Washington weighs possible drone strikes against the militants.
The ISIL fighters have taken control of a swath of territory north of Baghdad in a drive towards the Iraqi capital launched a week ago.
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The White House said in a statement that the US military personnel would help the State Department relocate some embassy staff from Baghdad to the consulates in Arbil and Basra, as well as Amman.
It added that the embassy remained open, and that most personnel were to remain in place in Baghdad.
The troops were entering Iraq with the consent of Prime Minister Nuri al-Maliki's government, the statement said.
