President Obama has hailed the support of Arab nations in air strikes on Islamic State (IS) militants, saying: "This is not America'sfight alone."
He was speaking hours after the US and Arab allies launched their first air strikes against IS in Syria.
Activists say at least 70 IS militants and 50 other al-Qaeda-linked fighters were killed in the strikes.
US state department spokeswoman Jan Psaki said the US had warned Syria in advance "not to engage US aircraft".
But she added that Washington had not requested permission or given advance notice of the timing of the attacks.
President Obama confirmed that Saudi Arabia, the United Arab Emirates, Jordan, Bahrain and Qatar had taken part in or supported the strikes.
He said the US was "proud to stand shoulder to shoulder with these nations".
'Imminent attacks' IS has seized large areas of Syria and Iraq, and the US has launched nearly 200 air strikes in Iraq since August.
But Monday's strikes expanded the anti-IS campaign across the border into Syria for the first time.
The strikes targeted the group's main headquarters in its stronghold of Raqqa, north-eastern Syria.
IS training compounds, vehicles and storage sites were also hit in several other areas.
The attacks were organised in three separate waves, Pentagon spokesman Lieutenant General Bill Mayville told reporters.
US fighter jets carried out the first set, with the Arab nations participating in the second and third waves, he added.
Later on Tuesday, US Secretary of State John Kerry said the US would not "allow geography or borders" to prevent action against IS militants.
"We will hold them responsible for their grotesque atrocities," he said at the United Nations in New York, flanked by Iraq's president and foreign minister.
President Obama said al-Qaeda-linked militants, known as the Khorasan Group, were also targeted by eight air strikes in Syria.
"We will not tolerate safe havens for terrorists who threaten our people," he said.
UN Secretary General Ban Ki-moon said that extremist groups operating in Syria "pose an immediate threat to international peace and security," but stopped short of endorsing the air strikes.
"I also note that the strikes took place in areas no longer under the effective control of that government," he told reporters at the UN headquarters in New York.
US officials say the group had been plotting "imminent attacks" against the West, and had established a safe haven west of Aleppo.
President Obama warned that the operation against IS would take some time, but that he would do "what is necessary" to defeat the extremists.
Secretary of State John Kerry said Turkey had pledged to take part in a US-led coalition against IS following the release of 46 Turkish hostages on Saturday, AFP reports.