WASHINGTON — Pentagon officials insisted Monday that the weekend’s Afghanistan killing spree was an “isolated incident” and said that a 38-year-old Army staff sergeant would soon be charged in connection with the deaths of 16 Afghan civilians, including nine children.

Defense Secretary Leon Panetta, traveling to Krygzystan, said Monday evening that the soldier could face the death penalty.

“We seem to get tested almost every other day with challenges that test our leadership and our commitment to the mission that we’re involved in,” Panetta said, according to an Associated Press reporter traveling with him. “War is hell.”

Under the Uniform Code of Military Justice, a premeditated murder conviction could carry the death penalty.

Regardless of what the soldier is charged with, defense and security experts worried Monday that the fallout from Sunday’s massacre in southern Kandahar province will be anything but isolated.

In the perception of Afghans, the experts said, the rampage merely adds to previous nightmares: the recent burning of Qurans by U.S. soldiers, eight children killed in February by NATO bombs, a video of Marines urinating on corpses, and others. Each incident separately posed problems for the Obama administration’s plans for an orderly end to the U.S. combat mission in Afghanistan, but the question now is whether, taken together, the incidents have made the American presence there untenable.

“The cumulative effect of these events makes it harder to climb what was already a very steep hill,” said John Nagl, an expert on military counterinsurgency strategy and a professor at the U.S. Naval Academy.

The pillars of the Obama administration’s plans to wind down the decade-long Afghan war seem increasingly shaky. As news of the killings sparked outrage in Afghanistan — prompting U.S. forces to beef up security precautions — it seemed increasingly unclear whether the United States could reach an agreement with the Afghan government for a long-term role for the American military, which U.S. officials are seeking to ensure that al-Qaida militants don’t regain safe haven there.

The incident also undermined U.S. efforts to push Taliban leaders into peace talks with the Afghan government, analysts said. As calls grow for U.S. forces to exit Afghanistan, a growing number of Afghans believe the Taliban are content to wait out the end of the American presence.

“Our strategy is based on building trust and goodwill,” Nagl said. “That trust takes a long time to build and can be destroyed by events like this very quickly.”

In Afghanistan, the killings fed into a narrative, promoted by the Taliban, that the U.S.-led international coalition frequently kills civilians. Afghan lawmakers condemned the shooting spree, saying that “Afghans have run out of patience with arbitrary acts of foreign forces,” according to a statement by the lower house of Parliament, which canceled its session Monday in protest over th e killings.

The Taliban wasted little time taking advantage of the situation, issuing a statement blasting the “sick-minded American savages.”

President Barack Obama told WFTV in Orlando, Fla., that the killings were “in no way … representative of the enormous sacrifices that our men and women have made in Afghanistan.” But he added, “It does signal the importance of us transitioning in accordance with my plans that Afghans are taking more of the initiative in security.”

A U.S. Army official, speaking on condition of anonymity because he wasn’t authorized to be quoted, said that the staff sergeant in custody would be charged by U.S. military authorities Kabul. The soldier, whose identity was being withheld pending the charges, is based at Joint Base Lewis-McChord in Washington state, officials said.

The staff sergeant had served three tours of duty in Iraq before arriving in Afghanistan, the official said — renewing questions about the strain of multiple deployments on U.S. service members over the past decade.

Pentagon spokesman George Little called the shooting an “isolated incident,” but Little has also described other recent incidents that inflamed U.S.-Afghan relations as isolated, such as the Quran burnings and the video of Marines urinating on corpses.

Steven Thomma of the McClatchy Washington Bureau and McClatchy special correspondent Ali Safi in Kabul contributed to this report.