"We will not tolerate safe havens for terrorists who threaten our people," Obama said as he left the White House to travel to the United Nations in New York, where he will meet officials from Arab nations that joined in the strikes against the Islamic State group.
The White House said US forces made separate strikes on its own against the Khorasan group, an al Qaeda affiliate, in order to disrupt planning for imminent attacks on the west.
"For some time now, we've been tracking plots to conduct attacks in the United States or Europe," Ben Rhodes, Obama's deputy national security adviser, told reporters traveling with the president to the United Nations General Assembly meeting.
"We believe that attack plotting was imminent," Rhodes said, "and that they had plans to conduct attacks external to Syria."
US and Arab strikes on militant targets in Syria overnight were just a start of a coalition effort to weaken Islamic State, an extremist group that has killed thousands and beheaded at least three westerners while seizing control of parts of Syria and northwestern Iraq.
"I can tell you that last night's strikes were only the beginning," Rear Admiral John Kirby, the Pentagon press secretary, told reporters. He said the strikes had been 'very successful' and would continue.
Another military spokesman said Arab nations, including Saudi Arabia, Jordan, Bahrain and the United Arab Emirates, took part in the second and third waves of attacks. Lieutenant General William Mayville Jr. said the actions by those Arab countries ranged from combat air patrols to hitting targets.
Mayville said the strikes hit training camps, headquarters, vehicles and other sites under the control of Islamic State. He said the strikes were the start "of a credible and sustainable persistent campaign to degrade and ultimately destroy" Islamic State.
Obama said he would meet with Iraqi Prime Minister Haider al-Abadi and "friends and allies" at the United Nations to continue building support for the coalition against the Islamic State group.
The US leader said the strength of the coalition, now at more than 40 countries, showed the fight against such militants is not America's alone.
"The overall effort will take time. There will be challenges ahead but we're going to do what's necessary to take the fight to this terrorist group," Obama said.
Obama said the campaign is in line with the strategy he outlined earlier this month to combat Islamic State.
The air campaign opened a new front against extremist groups that emerged and gained power while trying to topple President Bashar al-Assad during Syria's three-year-old civil war.
Syria's U.N. Ambassador Bashar Ja'afari said on Tuesday that he was personally informed by US Ambassador Samantha Power of the imminent US and Arab air strikes against Islamic State targets on Syrian territory hours ahead of time.
Ja'afari said Power told him on Monday morning that the military action would be carried out. He added that "we're in close coordination with Iraq."
The US mission confirmed that Power had informed Ja'afari.
