For the first time, militants from the group known as Islamic State or ISIL will be targeted in Syria by US forces.
It follows President Obama’s televised address setting out his strategy.
He promised “no safe haven” for any group that threatened America.
The extremists’ brutality, particularly the beheading of US journalists, has helped shift opinion decisively in favour of airstrikes.
“I have made it clear that we will hunt down terrorists who threaten our country, wherever they are. That means I will not hesitate to take action against ISIL in Syria, as well as Iraq,” the president said.
Many US airstrikes have already targeted militants in Iraq.
Some 475 more military personnel will now be sent to the country to help the 1,000 already there training local forces.
But no US troops will be taking part in combat missions. “We will not get dragged into another ground war,” Obama said.
In Syria he stressed the new strategy against ISIL would involve supporting the rebels fighting against President Bashar al-Assad.
“In the fight against ISIL, we cannot rely on an Assad regime that terrorises its people; a regime that will never regain the legitimacy it has lost. Instead, we must strengthen the opposition as the best counterweight to extremists like ISIL,” he said.
At the heart of the president’s plans is the formation of a broad coalition including Arab nations in the fight against the jihadists.
The Western-backed Syrian National Coalition has welcomed Obama’s plan, urging Congress to back it.
Last year opposition forced the president to back down over plans to launch airstrikes in Syria.
The president’s address came as his foreign policy ratings have plunged to their lowest yet, two months ahead of mid-term elections.
Euronews Washington correspondent Stefan Grobe gave his analysis of the president’s address:
“President Obama last night tried to walk a fine line between what seems to be a political and military necessity and what the US public is willing to tolerate during an election campaign.
“A large majority of Americans is still against any US combat mission in the Middle East. But at the same time, Americans want their president to respond forcefully to the ISIL threat.
“The strategy that Obama outlined reflects that: The jihadists will be destroyed with crucial US support, but ground combat must be fought by local forces.
“If necessary, this strategy would give Obama enough flexibility to step up the fight – after the November elections,” Grobe said.