The American military operation in parts of Iraq against the Islamic State terrorists is "limited in scope and duration", US President Barack Obama has informed the Congress.

In a letter to John Boehner, speaker of the US House of Representatives, on War Powers Resolution regarding Iraq, Obama said he has authorised the armed forces to conduct targeted airstrikes in Iraq.

"These military operations will be limited in their scope and duration as necessary to protect American personnel in Iraq by stopping the current advance on Erbil by the terrorist group Islamic State of Iraq and the Syria and to help forces in Iraq as they fight to break the siege of Mount Sinjar and protect the civilians trapped there," he said.