Meanwhile US warplanes stepped up air strikes against ISIS militants in Iraq and Syria.
“There can be no reasoning – no negotiation – with this brand of evil,” Barack Obama said.
The US president said more than 40 countries had offered to join the anti-IS coalition. ISIS aims to set up a hardline caliphate.
The well-armed Sunni Muslim militants have seized a huge swathe of Syria and Iraq, forcing whole communities to flee in terror. They have beheaded Western hostages and have persecuted Christians, Yazidis and Shia Muslims, whom they treat as heretics.
US warplanes hit ISIS vehicles and arms dumps in new air strikes, the US military’s Central Command said.
Eight ISIS vehicles were damaged near Abu Kamal on the Syria-Iraq border, and two others in Deir al-Zour in the east of Syria, the statement said.
In Iraq there were strikes on IS targets west of Baghdad and southeast of Irbil, near Kurdish territory.
Earlier there were air strikes on ISIS near the border with Turkey.
“The only language understood by killers like this is the language of force,” Barack Obama said.
The US “will work with a broad coalition to dismantle this network of death,” he told the UN.
“In this effort, we do not act alone. Nor do we intend to send US troops to occupy foreign lands. Instead, we will support Iraqis and Syrians fighting to reclaim their communities. We will use our military might in a campaign of air strikes to roll back ISIL [ISIS].
“We will train and equip forces fighting against these terrorists on the ground. We will work to cut off their financing, and to stop the flow of fighters into and out of the region. Already, over 40 nations have offered to join this coalition. Today, I ask the world to join in this effort.”
Barack Obama urged Muslims to reject the ideology of ISIS and al-Qaeda.
Syrian activists reported air strikes around the Kurdish town of Kobane near Turkey, which has been besieged by ISIS fighters for several days.
Witnesses saw two military aircraft approaching from Turkey but Turkish officials denied its airspace or bases had been used in the attack.
The US-led coalition expanded its raids against IS into Syria on Monday. The US said Saudi Arabia, the United Arab Emirates, Jordan, Bahrain and Qatar had all “participated in or supported” the strikes.
However, the aerial bombardment near Kobane, which happened at about 01:00 local time, has not been confirmed by the US or any coalition member.
Turkish military sources said neither its air force nor the US airbase at Incirlik in southern Turkey had been used.
The US has launched nearly 200 air strikes against ISIS in Iraq since August.
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