The White House said it was ready to "take action" against any threat to America as it indicated serious consideration of US military strikes against Islamic State militants in Syria.
The Pentagon has warned of the dangers of IS and said operations against it in Syria may be needed, after weeks of air strikes have slowed the advance of the Sunni militant group in Iraq and as the West reeled from the grisly killing of American journalist James Foley.
The White House also said late Friday that air strikes in Syria may be necessary, as deputy national security advisor Ben Rhodes agreed that "any strategy to deal with the ISIL organization has to deal with both sides of the border, Iraq and Syria."
"If we see plotting against Americans, we see a threat to the US emanating from anywhere, we stand ready to take action against that threat," Rhodes added.
"We've made very clear time and again that if you come after Americans, we're going to come after you wherever you are, and that's what's going to guide our planning in the days to come."
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A US military official also told the Wall Street Journal that strikes at so-called "high-value targets," such as individual leaders, could be mounted within "an hour, it could be a week."
"If it's based on training camps, we could do that pretty soon."
IS declared itself a "caliphate" in late June and has since added a swath of northern Iraq to territory it already held in eastern Syria.
US President Barack Obama launched a campaign of air raids to help regional Kurdish and Iraqi forces fighting IS in the country's north.
But if Washington were to launch military strikes in Syria, it would mean altering its current cautious approach to the civil war there.
Syria's civil war, which has been raging for more than three years, has thrown the region into turmoil and has so far killed more than 170,000 people.