Ten Arab countries have agreed to help the US in its fight against the jihadist group, Islamic State (IS).
After talks with US Secretary of State John Kerry in Jeddah, they pledged to provide military support and humanitarian aid, and to halt the flow of funds and foreign fighters to IS.
Mr Kerry told the BBC they were "full-throatedly ready" to combat the group.
However, Russia warned the US against expanding its campaign of air strikes from Iraq into neighbouring Syria.
The Russian foreign ministry said any such action, without the backing of the UN Security Council, would be "an act of aggression" and a "gross violation" of international law.
On Wednesday, President Barack Obama set out his plans to "destroy and degrade" IS and revealed that he had authorised air strikes in Syria.
Mr Kerry has been tasked with building a broad coalition against IS, also known as Isis or Isil.
On Thursday, ministers representing Bahrain, Egypt, Iraq, Jordan, Kuwait, Lebanon, Oman, Qatar, Saudi Arabia and the United Arab Emirates offered their support.
A joint communique declared a "shared commitment to stand united against the threat posed by all terrorism".
It added that participants had "discussed a strategy to destroy Isil wherever it is, including in both Iraq and Syria".
"The region recognises the danger that has been unleashed and they are full-throatedly ready to deal with that and that is why they committed today to take the actions they have committed to," Mr Kerry told the BBC.
Nato member Turkey was also at the Jeddah meeting, but did not sign the communique. Mr Kerry downplayed the move, saying the important US ally was dealing with some "sensitive issues" but remained "very engaged and has been very involved".
Analysis: Barbara Plett-Usher, BBC News, travelling with John Kerry
John Kerry was exhausted by his marathon day of meetings but pleased at its result: the Arabs' embrace of Barack Obama's strategy to combat Islamic State. It's crucial to the plan that Arab governments see this as much their war as America's.
Not everything went according to script. Turkey didn't sign the communique - because of "sensitive issues", said Mr Kerry, presumably the 49 Turkish hostages held by the militants - but was "very engaged". The Saudi foreign minister seems not to have heard that his country agreed to host the training of Syrian rebels as leaked by US officials.
But Mr Kerry was focused on the big picture: the region has recognised the danger in its midst and is ready to join the fight against it, he said; military officials will sort out of the details of who does what.
Turkey has reportedly been reluctant to take a prominent role in the coalition, partly out of concern for the 49 Turkish citizens being held hostage by IS.
Mr Kerry also rejected accusations that President Obama is responsible for the rise of IS because he failed to take action against the group earlier in the uprising against Syrian President Bashar al-Assad.
"I think over time people will realise the outgrowth of this is really the responsibility of Assad himself," he said. "He has been the magnet that attracted foreign fighters."
Mr Obama has asked the US Congress to approve a bill seeking $500m (£308m) to fund an increase in the training and arming of "moderate" Syrian rebels so that they can take the fight to the jihadist group.
The Republican Speaker of the House of Representatives, John Boehner, has given his support to the move, telling reporters: "At this point in time, it's important to give the president what he asked for."
In Iraq, the US has carried out more than 150 air strikes against IS.
On Thursday, aircraft attacked jihadist positions near the strategically important Mosul Dam in support of Iraqi security forces. The Pentagon said two machine-gun emplacements and a bunker were destroyed.
The US has also sent hundreds of military advisers to assist Iraqi government and Kurdish forces, but has ruled out sending ground troops.