The President vows to keep up the fight against the Islamic State after the US and five Arab nations launched airstrikes in Syria.
The US President spoke hours after the US and five Arab nations launched airstrikes against the group in Syria.
“America is proud to stand shoulder to shoulder with these nations on behalf of our common security,” Mr Obama said at the White House.
“The strength of this coalition makes it clear to the world that this is not America’s fight alone.”
He said defeating IS would take time and effort but added: “We’re going to do what’s necessary to take the fight to this terrorist group.”
After the brief statement, the President left for the UN General Assembly in New York, where he will seek to broaden consensus for the US-led effort.
The apparent aftermath of a US airstrike in Idlib
The American-led air campaign used land- and sea-based US aircraft as well as Tomahawk cruise missiles launched from two Navy ships in the Red Sea and the northern Persian Gulf.
At least 70 IS fighters are said to have been killed in dozens of attacks, which also targeted al Qaeda veterans.
The US has targeted IS facilities in Iraq but this is the first time the campaign has expanded into Syria, a country marred by three years of civil war.
The US military said it had destroyed or damaged multiple IS targets around the militant stronghold of Raqqa as well as Deir al Zor, Hasakah and the border town of Albu Kamal.
It said targets included IS fighters, training compounds, headquarters and command and control facilities, storage facilities, a finance centre, supply trucks and armed vehicles.
The five Arab nations – Saudi Arabia, Qatar, Bahrain, Jordan and the United Arab Emirate – either participated in the airstrikes or provided unspecified support, US officials said.
Jordan said its air force had bombed “targets that belong to some terrorist groups that sought to commit terrorist acts inside Jordan”.
Separately, the US alone carried out eight airstrikes to disrupt what the military described as “imminent attack plotting against the United States and Western interests” by a network of al Qaeda veterans.
“Once again, it must be clear to anyone who would plot against America and do Americans harm that we will not tolerate safe havens for terrorists who threaten our people,” said Mr Obama.
A US official said the al Qaeda-linked group was nearing the execution phase for an attack in Europe or the US.
Mr Obama had been wary of dragging the US military into the conflict between the regime of President Bashar al Assad and rebel groups.
However, Damascus says the US had informed Syria’s envoy to the UN about the strikes.
Activists said the airstrikes hit targets in and around the Syrian city of Raqqa and the province with the same name.
Raqqa is the Islamic State group’s self-declared capital in Syria.
International efforts to combat the group have taken on an added urgency after the beheading of US journalists James Foley and Steven Sotloff and British aid worker David Haines, and the threat to kill UK hostage Alan Henning.
The strikes did not involve the UK but Prime Minister David Cameron supported them and will discuss at the UN what contribution Britain can make, according to Downing Street.
Photographs taken in Raqqa showed wreckage of what IS fighters said was a drone that had been shot down.
Pieces of the wreckage were shown loaded into the back of a van.