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.
 Car used by IS fighters is destoyed in a US airstrike  
 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 was kidnapped in Syria two years ago  
 Mr Foley's killer is believed to be British  
 The US has launched 57 airstrikes around Mosul Dam  
 Chuck Hagel warns of the threat posed by IS  
 Pope Francis has phoned Mr Foley's parents Diane and John  
 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  
 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  
 Michael Brown was shot dead on August 9  
 The teen's shooting death has sparked daily protests  
 Kurdish peshmerga fighters have reportedly regained control of Mosul dam  
 Peshmerga fighters on the way to the Mosul dam  
 A woman, seen via a shrapnel hole, reacts after shelling in Donetsk