The US has said it will not be restricted by the Iraq-Syria border as it considers further action against Islamic State militants.
White House deputy national security adviser Ben Rhodes the US stood ready to take action to protect American citizens as the group was more dangerous now than it was six months ago.
The US has already carried out airstrikes on the group - formerly known as ISIS or ISIL - in Iraq as it has sought to support government forces and Kurdish Peshmerga in their attempts to push back the jihadists.
However, it has so far steered clear of Syria, except for a brief special forces raid which attempted to rescue journalist James Foley and other American hostages.
Red shows areas controlled by IS, while yellow is areas of fighting Mr Foley was beheaded by a member of the group - believed to be British - in a video released earlier this week. The black-clad militant said the journalist was killed in retaliation for US airstrikes.
"When you see somebody killed in such a horrific way, that represents a terrorist attack- that represents a terrorist attack against our country and against an American citizen," Mr Rhodes told reporters at the White House
He added that the US had done everything it could to rescue American hostages but would keep trying to get back those still held by the group.
The announcement that the US would consider acting in Syria came after the former head of the British Army said the West should consider negotiating with Syrian president Bashar al Assad to tackle IS.
Lord Dannatt told Sky News: "You have to at least consider the otherwise unpalatable thought that maybe we've got to have some kind of dialogue, whether it's under the counter or over the counter, with President Assad of Syria.
Journalist James Foley was beheaded on video by an IS militant "The old dictum that my enemy's enemy is my friend just might have some credence in this less than satisfactory and pretty extraordinary set of times that we are in."
However, UK Foreign Secretary Philip Hammond ruled out working with Mr Assad - who is accused of carrying out war crimes, including chemical attacks, during his country's three-year civil war.
He said Britain would help Kurdish and Iraqi forces with weapons and training once there was a credible government in place in Baghdad.
However, efforts to form a new government around Prime Minister-designate Haider al Abadi were dealt a major blow when Sunni politicians pulled out of talks following an attack on a mosque that killed at least 64 people and injured 60 others.
The mass killing at Friday prayers was initially blamed on Shia militia allied with the government but there have also been suggestions that IS fighters, who have been trying to recruit Sunni tribes in the area, could have staged the attack.
Kurdish Peshmerga are leading the fightback against IS in Iraq Elsewhere, Kurdish forces have launched a major assault to try to retake the northeast Iraqi towns of Jalula and Sadiyah.
Sky's Alex Crawford, reporting from the outskirts of Jalula, said the operation was being carried out by the Kurdish military's elite counter-terrorism unit, backed up by peshmerga forces.
She said the towns, near the Iranian border and semi-autonomous Kurdish region, had been under IS control for more than two months.
"What is significant about this assault is that they (the Kurds) are doing this pretty much entirely on their own," she said.
"They've had very little air support. There is no evidence of any outside weaponry, military hardware to back them up."
Although US Defense Secretary Chuck Hagel has warned that IS is the most dangerous threat faced by America for years, the FBI on Friday said there were no specific or credible threats that the group was planning an attack on US soil.
An intelligence bulletin, issued to state and local law enforcement, said officials were concerned though that IS supporters could attack overseas targets with little warning.
PM designate Haider al Abadi is trying to form a more inclusive government
Diyala has seen heavy fighting between Iraqi troops and IS (file pic)
Peshmerga are fighting to gain territory from militants north of Baghdad
The US says the killing of James Foley was a "terrorist attack" on America
Ukraine insisted the aid convoy entered under the auspices of the Red Cross
President Poroshenko is due to meet Russia's leader next week
Ukraine said it captured two Russian armoured vehicles
Sources said Germany's Angela Merkel would push for a ceasefire
The 280-truck convoy departed from near Moscow more than seven days ago
James Foley in Libya in 2011
A rough outline of the caliphate declared by IS militants
Militants vowed to attack US targets in another video clip
James Foley was working as a journalist covering the war in Syria
The killer spoke with a British accent
US President Obama says the US will do whatever is necessary
James Foley disappeared in Syria in November 2012. Pic: Nicole Tung
David Cameron has returned from his holiday in the wake of the killing
Private security guards may be set to return to Iraq. Pic: Pete Norman
Mr Foley in Syria before he was captured. Pic: Nicole Tung
John and Diane Foley, Mr Foley's parents, helped a public campaign 
A woman, seen via a shrapnel hole, reacts after shelling in Donetsk