But in a sign of their growing strength, a monitoring group said the jihadists had managed to bring down a Syrian warplane conducting strikes over their stronghold of Raqa in north-central Syria.
The US air strike against IS fighters in the Sadral-Yusufiyah area, 25 kilometres from Baghdad, came as world diplomats pledged to support Iraq in its fight against the militants and less than a week after US President Barack Obama ordered a "relentless" war against IS.
"US military forces continued to attack (IS) terrorists in Iraq, employing attack and fighter aircraft to conduct two air strikes Sunday and Monday in support of Iraqi security forces near Sinjar and southwest of Baghdad," the US Central Command said in a statement.
"The air strike southwest of Baghdad was the first strike taken as part of our expanded efforts beyond protecting our own people and humanitarian missions to hit (IS) targets as Iraqi forces go on offence, as outlined in the president's speech last Wednesday," the statement read.
The strikes destroyed six IS vehicles near Sinjar and an IS position southwest of Baghdad that had been firing on Iraqi forces.
They bring the number of US air strikes across Iraq to 162.
Iraqi security spokesman Lieutenant General Qassem Atta on Tuesday welcomed the expanded American action, saying the US "carried out an important strike against an enemy target in Sadr al-Yusufiyah."
Sadr al-Yusufiyah lies in the Euphrates Valley, between the militant stronghold of Fallujah, west of Baghdad, and the key battleground of Jurf al-Sakhr, further south.
It is one of the closest front lines to Baghdad where Iraqi government forces and allied militia have struggled to defend their positions.
IS militants have seized a swathe of territory in Iraq and Syria, declaring an Islamic "caliphate", committing widespread atrocities and instituting a brutal interpretation of Islamic law.
As part of the extended campaign Washington has vowed to carry out strikes in Syria as well, despite warnings from President Bashar al-Assad's regime against violating its airspace.

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