Thirty countries have pledged to help Iraq fight Islamic State (IS) militants "by all means necessary".
A joint statement by foreign ministers taking part in a major conference in Paris said support would include "appropriate military assistance".
The talks had been called to agree a strategy to combat the group, which controls large parts of Iraq and Syria.
The meeting followed a whirlwind tour of the Middle East by US Secretary of State John Kerry.
Mr Kerry, who attended the summit, has been drumming up support for a plan of action unveiled by President Barack Obama last week.
The CIA estimates that Islamic State has between 20,000 and 31,000 fighters in Iraq and Syria.
The murder of British aid worker David Haines by IS militants, shown in a video released by the group on Saturday, has added momentum to the plans, says the BBC's Lucy Williamson in Paris.
Analysis: Jeremy Bowen, BBC News, Damascus
Another layer of conflict is being grafted on to a series of parallel and overlapping wars in Syria and Iraq.
Enemies of the Syrian regime, including Saudi Arabia, will want to calculate how much their actions against Islamic State might strengthen President Bashar al-Assad - who has men tied up fighting IS.
Iran and Saudi Arabia, regional superpowers, back opposing sides in Syria. They will look very closely to see if American actions indirectly help their rival.
Some Islamist fighters in Syria who have been trained and armed by Saudi Arabia or Qatar have already gravitated to IS. Opposition to Western involvement might make more follow.
And the US and its Western allies are becoming direct players in the wars in Syria and Iraq - and in the deepening sectarian conflict between Shia and Sunni Muslims.
Many argue that the Americans and the British tore open sectarian scars when they invaded Iraq in 2003, and now risk making matters even worse.
Iraqi President Fuad Masum, who co-hosted the conference with French President Francois Hollande, said the international community must pursue the jihadists "quickly".
"If this intervention and support to Iraq is late, that means that Islamic State could occupy more territory and the threat it poses will be even bigger," he said.
The summit declaration said participants were "committed to supporting the new Iraqi government in its fight... by any means necessary, including appropriate military assistance".
Iraqi Foreign Minister Ibrahim Jafari welcomed the decision, but also said he regretted that Iran was not present at the Paris talks.
Last week Mr Kerry ruled out co-operation with Iran citing its "engagement in Syria and elsewhere".
But Iranian Supreme Leader Ayatollah Ali Khamenei claimed on Monday that the US had requested Iran's co-operation via the US Ambassador to Iraq.
"I said no, because they have dirty hands," he said. He added the US was seeking seeking a "pretext to do in Iraq and Syria what it already does in Pakistan - bomb anywhere without authorisation".
Syria also did not take part in the Paris gathering.
'Doing their share'
Meanwhile, France said it had begun surveillance flights over Iraq. Britain revealed in August that its aircraft had been gathering intelligence over Iraq.
Several Arab countries have offered to take part in air strikes on IS fighters in Iraq, US officials say.
Turkey, however, will only allow humanitarian and logistical operations from the Nato air base on its soil.
Mr Kerry said he was "extremely encouraged" by promises of military assistance to tackle the militant group.
The US strategy to weaken the group centres on military support for Iraq but also includes plans to stop foreign fighters from joining the group, cutting its funding streams and trying to counter its ideology.
The Paris conference was aimed at defining the role each member state will play.
About 40 countries have so far signed up to a coalition including 10 Arab states - Egypt, Iraq, Jordan, Lebanon, Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the United Arab Emirates.
Australia announced at the weekend that it was sending 600 troops and up to eight fighter jets to the UAE ahead of possible combat operations in Iraq.
However, Mr Kerry told US broadcaster CBS that the US was not seeking troops on the ground at the moment.
Since August, US fighter jets have conducted about 160 air strikes on IS positions in Iraq.
The group's former name was Isis and it has also been known as Isil (Islamic State in Iraq and the Levant).