America may launch a broader military campaign that targets an Islamic extremist group’s bases in Syria.
A senior White House official raised the possibility, saying the US would take whatever action is necessary to protect national security.
“We’re not going to be restricted by borders,” said Ben Rhodes, President Barack Obama’s deputy national security adviser.
The White House said the president has received no military options beyond those he authorised earlier this month for limited airstrikes against the Islamic State (IS) group in Iraq and military aid to Iraqi and Kurdish forces.
The US has avoided military involvement in Syria’s three-year civil war.
But faced with the Islamic State making gains across the region and the beheading of American journalist James Foley, the administration’s resistance may be weakening.
Mr Rhodes spoke a day after Mr Obama’s senior military adviser warned the extremists cannot be defeated without “addressing” their sanctuary in Syria.
Many prominent Republicans and some Democrats have called on Mr Obama to hit back harder at the Islamic State militants.
Senator Marco Rubio, a prospective 2016 presidential candidate, said that attacking their supply lines, command and control centres and economic assets inside Syria “is at the crux of the decision” for Mr Obama.
The risk of “getting sucked into a new war” is outweighed, he said, by the risk of inaction.
To hit back at the group, Obama has stressed military assistance to Iraq and efforts to create a new, inclusive government in Baghdad that can persuade Sunnis to leave the insurgency.
He also has sought to frame the IS threat in terms that convince other countries – not just in the Middle East but also in Europe – of the need to create a broad coalition against the extremists.
Mr R hodes said the US was “actively considering what’s going to be necessary to deal with that threat”.
He added: “We’ve shown time and again that if there’s a counter-terrorism threat, we’ll take direct action against that threat, if necessary.”
The murder of Mr Foley could be seen as a turning point in a long-running battle against the group, whose origins are in an al Qaida offshoot that US forces faced in Iraq several years ago, he said.
Mr Foley’s killing, he stressed, was “an attack on our country”.