The remotely-piloted Reaper planes will provide surveillance, reconnaissance and intelligence support to the Iraqi troops and international coalition forces taking on the IS group in northern Iraq.
They can also launch bombs and missiles.
It will be the first time Britain has deployed Reapers outside of Afghanistan, where Britain is completing a pull-out of combat troops this year.
"We are in the process of re-deploying some of our Reaper remotely-piloted aircraft from Afghanistan to the Middle East," Hammond said.
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"The surveillance capability of Reaper will see it provide vital situational awareness, making it an invaluable asset to the Iraqi government and the coalition allies," Defence Secretary Michael Fallon told parliament.
"If strike operations are required then Reaper has the ability to complement the sorties RAF Tornados have already completed."
The US-made Reapers are normally armed with two Paveway laser-guided bombs and four Hellfire missiles for precision strikes.
The defence ministry also said a small group of British infantry have completed a week training the Kurdish forces fighting extremists in using the heavy machine guns Britain gave them last month.