Even as an emboldened United States said it would launch air strikes in Syria to crush militants, it invited criticism that it was overstepping its boundaries.
Both the Chinese and Russian foreign ministries have issued statements saying that the fight against militants in the Middle East still needs to conform with international laws.
Said Chinese Foreign Ministry spokesman Hua Chunying: "We hold that in the international struggle against terrorism, international law should be respected, as well as the sovereignty, independence and territorial integrity of the countries concerned."
Syria's government has also reacted strongly to US plans to launch air strikes on ISIS targets there for the first time, warning Washington that any operation without its consent would be deemed an "act of aggression".
In a prime-time televised address on Wednesday night, President Barack Obama shed his cautious approach and announced a comprehensive strategy meant to take the fight to the Islamic State in Iraq and Syria terror group, also known as ISIL.
"I have made it clear that we will hunt down terrorists who threaten our country, wherever they are. That means I will not hesitate to take action against ISIL in Syria as well as Iraq. This is a core principle of my presidency: If you threaten America, you will find no safe haven," he said.
His tough, new stance - which reflects a shift in US public sentiment after ISIS beheaded two American journalists - struck a chord at home and with the country's allies.
Syria's opposition coalition president Hadi al-Bahra said it was "willing to partner with the international community not only to defeat ISIS but also to rid the Syrian people of the tyranny of the Assad regime". Saudi Arabia said it had agreed to host training camps for moderate Syrian rebels.
The 15-minute speech at the White House on Wednesday night was vintage Obama. The four prongs of his strategy - deployed as part of a broad coalition - were air power, supporting local forces, using counterterrorism tactics and providing humanitarian aid.
He tried to differentiate the current offensive from painful wars that the US had been embroiled in over the past decade.
"It will not involve American combat troops fighting on foreign soil. This counterterrorism campaign will be waged through a steady, relentless effort to take out ISIL wherever they exist. This strategy... is one that we have successfully pursued in Yemen and Somalia for years."
Over the coming days, Secretary of State John Kerry is set to travel across the region and Europe to find others willing to contribute troops or funds to the fight. In the US, the Department of Homeland Security will be working to track ISIS foreign fighters. The Treasury Department will seek to disrupt the group's funding from external sources.
Mr Obama wants lawmakers to authorise a US$500 million (S$632 million) programme to support Syrian rebels.
Some Republicans seem willing to give the plan a chance.
Said House Speaker John Boehner: "He has finally begun to make the case that the nation has needed him to make for quite some time: that destroying this terrorist threat requires decisive action and must be the highest priority for the US and other nations of the free world."