Ten states, including heavyweight Saudi Arabia, "agreed to do their share in the comprehensive fight" against Islamic State, said a statement after a meeting between US Secretary of State John Kerry and his Arab counterparts.
With Iraq's new unity government and the Syrian opposition welcoming Obama's plan, Syrian President Bashar al-Assad's regime and powerful ally Russia strongly condemned it.
"Any action of any kind without the consent of the Syrian government would be an attack on Syria," National Reconciliation Minister Ali Haidar said.
"Our objective is clear: we will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy," Obama said in a television address, using an alternative acronym for the group.
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"I will not hesitate to take action against ISIL in Syria, as well as Iraq."
Obama announced the dispatch of another 475 military personnel to help train Iraqi forces to take on IS, bringing the total in the country to 1,600.
Instead, Washington is looking to empower partners on the ground like Iraqi and Kurdish forces, as well as Syrian rebels, to fill in territory opened up by its air power.
Key to that will be improving the effectiveness of the rebels, and Obama called on Congress to swiftly authorise an operation to train and equip moderate fighters.