File photo of US President Barack Obama. (AP)
The United States said on Tuesday that it would not coordinate with the government of President Bashar al-Assad on targeting Islamic State militants on Syrian territory, as preparations for possible air strikes gathered pace.
President Barack Obama meanwhile warned that it would not be a quick or easy job to root out the "cancer" of IS, but said extremists responsible for the murder of US journalist James Foley would be hunted down and America would not forget their crime.
Obama has already authorized spy flights over Syria to prepare for possible attacks on IS to match those already conducted against the jihadist group in Iraq, prompting the Assad regime to warn Washington would need permission to operate in Syrian airspace.
But White House spokesman Josh Earnest said Washington did not even recognize Assad -- whom it blames for igniting a vicious civil war -- as the legitimate leader of Syria, and dismissed his government's offer of a joint effort to combat "terrorism."
"There are no plans to coordinate with the Assad regime as we consider this terror threat," Earnest said.
Obama, speaking to military veterans in North Carolina, offered prayers to the parents of Foley, who was murdered by IS last week, and said justice would be done.
"Our message to anyone who harms our people is simple: America does not forget. Our reach is long. We are patient. Justice will be done," Obama told the American Legion annual convention.
"We have proved, time and time again, we will do what's necessary to capture those who harm Americans, to go after those who harm Americans."
IS a 'cancer'As he contemplates military action against IS in Syria, Obama also warned against expecting immediate results as US intelligence agencies begin to build data on potential targets.
"Rooting out a cancer like (IS) won't be easy and it won't be quick," he said.
Obama also defended his decision to bring tens of thousands of US troops home from Iraq and to end combat operations in Afghanistan by the end of this year. He reiterated that America would not be "dragged" into more fighting by deploying ground troops in Iraq, despite recent air strikes.
"Ultimately, it is up to the Iraqis to bridge their differences and secure themselves," Obama said, arguing that US air strikes in Iraq over the last two weeks had protected US advisors there and helped local forces advance against IS.