This indicates that the videos were probably made at different times, as does an apparent reference by the masked man to American airstrikes near the Iraqi town of Amerli, which the military carried out last weekend. Mr. Obama did not address the killing before leaving for Estonia, where he plans to reassure Baltic NATO allies in the face of Russia’s incursions into Ukraine. Administration officials said he did not want to speak before intelligence agencies had authenticated the video.

White House officials said that Mr. Obama remained cautious about military strikes in Syria and that he was focused for now on developing a strategy and assembling a broad coalition of countries to deal with ISIS, one of several combatants in Syria’s three-year-old civil war.

But the harrowing images of Americans with knives to their throats have given the threat from ISIS an emotional resonance and stoked calls on Capitol Hill and elsewhere for Mr. Obama to act more boldly. Some current and former counterterrorism officials said that although Mr. Obama would be under extraordinary pressure to retaliate, there were arguments against striking back.

“That pressure is often the enemy of good policy,” said Daniel Benjamin, a former State Department counterterrorism coordinator and now a scholar at Dartmouth College. “There will be a clamor for the president to take military action, which may not be effective. If he conducts airstrikes and does not get the desired effect, there’ll be pressure for more airstrikes, and then to put boots on the ground.”

Senator Bill Nelson, a Florida Democrat and member of the Senate Armed Services Committee, said he would introduce a bill giving Mr. Obama authority to order airstrikes in Syria. Republicans and even some Democrats have criticized the president’s admission last Friday that “we don’t have a strategy yet” for combating ISIS there.

Image A screen grab from a video shows American Steven J. Sotloff, 31, just before the masked person appeared to behead him.

At the same time, some experts warned that it would be difficult to mount a rescue mission like the one that Army Delta Force commandos tried in July, when they raided an oil refinery near the Syrian city of Raqqa after receiving information that American hostages, and possibly others, were being held there. By the time the commandos arrived, the hostages were gone.