
The military is "using a mix of fighter, bomber and Tomahawk Land Attack Missiles," Pentagon Press Secretary Rear Admiral John Kirby said in a statement.
The decision to conduct the airstrikes was made earlier Monday by US Central Command under authorization from US President Barack Obama.
The bombings are the first against Islamic State militants in Syria.
The US had previously bombed Islamic State targets in Iraq, but said that it would pursue the group in Syria if necessary.