U.S. President Donald Trump and his top national security aides huddled in the seclusion of Camp David on Friday, trying to formulate an effective new Afghanistan strategy after 16 frustrating years of war.

The administration has struggled for months to fashion a plan for turning around the stalemated war. But stepping up the fight in a way that advances peace prospects may be even more difficult.

Trump was meeting with Secretary of State Rex Tillerson, Defense Secretary Jim Mattis and other top military and diplomatic aides at the presidential retreat in nearby Maryland. Mattis said earlier this week the administration was "very close" to finalizing a new approach.

The meeting participants did not include Steve Bannon, the Trump strategist who has clashed with other members of the national security team over how to proceed in Afghanistan. His resignation was announced at midday.

By retreating to the seclusion of Camp David in Maryland's Catoctin Mountains, Trump was taking an opportunity to regroup after a politically bruising week of criticism of his response to the deadly protests in Charlottesville, Virginia.

Solutions for Afghanistan, the longest war in American history, eluded the Obama administration and haven't come easily to Trump, who said almost nothing about the conflict during his presidential campaign. Since taking office, he has considered options ranging from walking away from the war to sending in additional troops. Abandoning Afghanistan is seen as unlikely in light of U.S. concerns about countering terrorism.

In remarks at the State Department on Thursday, Mattis told reporters the Camp David talks "will move this toward a decision."

"We are coming very close to a decision, and I anticipate it in the very near future," he added.

Months ago the Pentagon settled on a plan to send about 3,800 additional troops to strengthen the Afghan army, which is stuck in what some call a deteriorating situation with the Taliban insurgency. Within in the White House, questions persist about the wisdom of investing further resources in the war.

The administration has said its Afghanistan strategy will be informed by a review of its approach to the broader region, including Pakistan and India. The Taliban have long used Pakistan as a sanctuary, complicating efforts to defeat the insurgency in Afghanistan and stabilize the country. The region includes other actors who pose political problems for Washington, including Iran, which has influence in western Afghanistan.

The outlook is clouded by the Afghan government's struggle to halt Taliban advances on its own. The U.S. Special Inspector General for Afghanistan Reconstruction has said the Taliban hold sway in almost half the country.

Government forces also are battling a Daesh affiliate that has carved out a foothold mostly in the east. Trump has vowed to crush Daesh, so its expansion in Afghanistan poses an additional challenge with no immediate solution. Just this week, a U.S. soldier was killed and nearly a dozen were wounded in combat with Daesh fighters.

The U.S. has about 8,400 troops in Afghanistan. Their primary roles are to train and advise Afghan forces and to hunt down and kill members of al-Qaida and other extremist groups.

Trump has expressed frustration at the prolonged fighting in Afghanistan. Earlier this summer he raised the idea of firing the top U.S. commander there, Gen. John Nicholson.

Asked this week if Trump has confidence in Nicholson, Mattis demurred. "Ask the president," he answered.

Trump is "looking at all aspects" of U.S. involvement in Afghanistan "as he must in his responsibilities as the commander in chief," Mattis said.

Lawmakers in Congress also are frustrated by the war and the prolonged debate within the administration on how to break the stalemate. Last week, Republican Sen. John McCain declared that "America is adrift in Afghanistan." He proposed a war strategy that would expand the U.S. counterterrorism effort and provide greater support to Afghan security forces.

McCain said bluntly, "We are losing in Afghanistan, and time is of the essence if we intend to turn the tide."