U.S. President Barack Obama has not yet made a decision to pursue military options in Syria against Islamic State extremists.
White House spokesman Josh Earnest Monday said that while the U.S. was serious about confronting the Islamic State threat, military force was not the "only tool in the toolbox."
He added that the only sustainable solution would require the active involvement of an effective and inclusive Iraqi government.
Earnest said there was no evidence of an active plot under way by the Islamic State against the U.S. homeland.
Earlier Monday, Syria said it was willing to work with the international community, including the U.S. and Britain, to fight the advance of Islamic State militants inside Syria, but it warned that any attacks should only be carried out in coordination with Damascus.
The U.S. already is carrying out extensive airstrikes with fighter jets and drones against Islamic State fighters in northern Iraq. The attacks have helped Kurdish soldiers retake the key Mosul Dam that supplies electricity and irrigation to a large swath of Iraq.
FILE - U.S. journalist James Foley, center, arrives in Tripoli after being released by the Libyan government, May 18, 2011.
Late last week, after an American journalist was beheaded by the militants, Washington said it was considering expanding its operations to attack Islamic State positions inside neighboring Syria.
Foreign Minister Walid al-Moallem said Monday Syria was ready to help enforce a United Nations Security Council resolution approved earlier this month aimed at cutting funding and the flow of foreign fighters to the Islamic State and al-Qaida's Syrian affiliate, the al-Nusra Front.
But there is no sign that Washington is easing its opposition to Syrian President Bashar al-Assad, who has maintained power during a bloody three-year fight against insurgents, including Islamic State militants, seeking to overthrow him.
Moallem said any unilateral action by the United States inside Syria "would be an act of aggression." In that event, he warned that Syria's air defenses could attempt to shoot down U.S. warplanes.
Ambassador James Woolsey - former director of the U.S. Central Intelligence Agency during the Clinton administration and now the chairman of the Foundation for Defense of Democracies in Washington, says that "paying particular attention to one executioner is only of modest importance.”
Woolsey said “It is a terrible thing that he did. It would be great to have intelligence about exactly where he is, and perhaps be able to let a predator fire a hellfire into his lap,” but he explained it is not a high strategic priority.
The ambassador explained the United States was fighting a movement not an individual.
An Islamic State militant uses a loud-hailer to announce to residents of Taqba city that Tabqa air base has fallen to Islamic State militants, in nearby Raqqa city, Aug. 24, 2014.
The Islamic State group has taken over much of eastern Syria and northern and western Iraq as it seeks to impose an Islamic caliphate without regard to national borders.
The group's fighters have continued to advance, seizing territory from both armed opposition groups in Syria's northern Aleppo province and from the Syrian army in northern Raqqa province. On Sunday, the Islamic State took control of Tabqa air base, the last Syrian military outpost in Raqqa.
Meanwhile, U.N. human rights chief Navi Pillay accused the Islamic State of carrying out "widespread ethnic and religious cleansing," targeting people based on their ethnic, religious or sectarian affiliation. She said such persecution amounted to crimes against humanity.