Two B-52 heavy bombers took off from Fairford airbase in western England to launch new raids in Iraq.
The first B-52 left the base in Gloucestershire shortly after 11.30 p.m. (Irish time) on Saturday, with the second taking off about 30 minutes later.
The B-52 is one of the world's most powerful bombers.
With a wingspan of nearly 60 metres and eight massive engines, it can carry nuclear or conventional weapons and launch as many as 20 cruise missiles at once.
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The Americans used the aircraft to devastating effect in the Vietnam war and the planes were responsible for breaking the back of Iraq's elite Republican Guard during the 1991 Gulf War.