The United States is considering taking the fight against Islamic State militants into Syria after days of airstrikes against the group in Iraq and the beheading of an American journalist, the White House signaled on Friday.

President Barack Obama, soon to end a two-week working vacation on the Massachusetts island of Martha's Vineyard, has not yet been presented with military options for attacking Islamic State targets beyond two important areas in Iraq, said White House deputy national security adviser Ben Rhodes.

But Rhodes made clear that going after Islamic State forces based in Syria is an option after the release of a video this week showing one of the group's fighters beheading American journalist James Foley and threatening to kill a second American, Steve Sotloff.

"We will do what's necessary to protect Americans and see that justice is done for what we saw with the barbaric killing of Jim Foley. So we're actively considering what's going to be necessary to deal with that threat, and we're not going to be restricted by borders," he said.

The sophistication, wealth and military might of Islamic State militants represent a major threat to the United States that may surpass that once posed by al Qaeda, US military leaders said on Thursday.

"They are an imminent threat to every interest we have, whether it's in Iraq or anywhere else," Defense Secretary Chuck Hagel told reporters at the Pentagon.

Hagel's assessment of Islamic State, which gained strength during Syria's civil war and swept into northern Iraq earlier this summer, sounded a note of alarm several days after the group posted a video on social media showing one of its fighters beheading an American hostage kidnapped in Syria.

Asked if the hardline Sunni Muslim organization posed a threat to the US comparable to that of the attacks of Sept. 11, 2001, Hagel said it was "as sophisticated and well-funded as any group we have seen."

"They are beyond just a terrorist group. They marry ideology, a sophistication of ... military prowess. They are tremendously well-funded. This is beyond anything we've seen."

Hagel spoke as the United States continued attacking Islamic State targets in Iraq.

The US effort against Islamic State thus far has been relatively limited. US forces have conducted more than 90 airstrikes in Iraq to protect the Iraqi Yazidi religious minority and attack Islamic State positions around the Mosul Dam.

Extending the fight into Syria would allow opportunities for disrupting the group's supply lines. Republican Senator John McCain told Reuters this week that Islamic State fighters have moved military equipment seized in the northern Iraqi city of Mosul into Syria and that they hold enclaves in Syrian territory that have been identified.

A move into Syria, even only with air power, would be a reversal for Obama. He stepped back from a threat to launch airstrikes in Syria a year ago in response to a chemical weapons attack against civilians it blamed on forces loyal to Syrian President Bashar al-Assad.

Time and time again over the past year, Obama has rejected greater involvement in the three-year-old Syrian civil war out of concern about getting entangled in a conflict with no clear positive outcome for the United States.

But officials say the situation now is different because Islamic State militants represent a direct threat to Americans and American interests. General Martin Dempsey, chairman of the US Joint Chiefs of Staff, said on Thursday that Islamic State cannot be defeated without addressing the part of the group that is based in Syria.

US forces have already had one direct ground engagement with Islamic State militants in Syria.

The White House said this week that a rescue mission this summer aimed at securing the freedom of Foley, Sotloff and a handful of other American hostages led to a firefight in which a number of militants were killed. The hostages were not at the location.

The United States is taking the Islamic State militants far more seriously now than it did six months ago, when Obama told the New Yorker magazine that they were the "JV team," which is short for "junior varsity" and means they are not the best players on the field.

Rhodes said the group is far more dangerous now than it was six months ago.-Reuters