British fighter jets took off from RAF Akrotiri in Cyprus early on Thursday shortly after Britain's parliament voted to bomb Islamic State targets in Syria, a Reuters witness said.
Four jets left in pairs within about an hour of each other from the airbase. Their destination was not immediately clear and there was no comment from British authorities.
RAF Akrotiri has been used as a launchpad for attacks on Islamic State targets in Iraq for just over a year, and late on Wednesday Britain's parliament broadened its scope for targets within Syria.
After more than 10 hours of tense debate, British lawmakers voted in favour of the air strikes, by 397 to 223.
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