President Donald Trump has given the Pentagon new authorities to decide the troop levels in Afghanistan, a U.S. official said Tuesday. The move could lead to a deployment of thousands more troops as commanders decide the way forward in the now 15-year-old war.

The U.S. official, who spoke on the condition of anonymity to speak more freely, said the move is similar to the April decision that gave the Pentagon more authority to set troop levels in Iraq and Syria. The change, the Pentagon said, was so that units could deploy at their proper strength to better maintain unit cohesion.

With the new authority, Secretary of Defense Jim Mattis could authorize deployment of additional troops to Afghanistan, something commanders on the ground there have been requesting for months. Gen. John Nicholson, the commander of U.S. forces in Afghanistan as well as his direct superior, head of U.S. Central Command, Gen. Joseph Votel, have both made cases for sending a "few thousand" more troops. If sent, the forces would help the fledgling Afghan military regain portions of the country that have fallen to the Taliban since U.S. forces ended their combat mission there in 2014.

The decision from the White House comes the same day Mattis told lawmakers on the Senate Armed Services Committee that "we are not winning" in Afghanistan. Mattis said the Taliban was surging throughout the country and that he planned to present lawmakers with a strategy for the United States' longest running war by mid-July.

Incensed, chairman of the committee, Sen. John McCain, R-Ariz., said Congress couldn't pass a budget without a strategy.

"We can't keep going like this," McCain said. "We know what the strategy was for the last eight years: Don't lose. That hasn't worked."

When asked what "winning looks like," Mattis replied that it would mean a long-term U.S. presence and Afghan security forces that were capable enough to control violence at local levels.

"It's going to be an era of frequent skirmishing and it's going to require a change in our approach from the last several years if we're to get it to that position," Mattis said.

In the short-term, Mattis and the chairman of the Joint Chief's of Staff, Gen. Joseph Dunford, said additional U.S. troops sent to the country would provide more fire and air support to the Afghans. Airstrike and artillery, they reasoned, would give the Afghan forces breathing room to build a more effective force.

In the first eight months of 2016, Afghan forces suffered 15,000 casualties including more than 5,000 killed. Recruiting efforts have been barely able to keep the Afghan security forces from maintaining their current ranks, let alone growing to a size large and capable enough to project security in the country.

The Taliban "had a good year last year," Mattis said.

With an air force that is still in its infancy and corruption still rampant in the ranks, some experts think it could take years for the Afghan forces to mature enough to negate the U.S. role in the country.

There are currently around 8,400 U.S. troops in Afghanistan and roughly 5,000 additional NATO forces in the country. The U.S. contingent is currently split between conducting counterterror operations alongside Afghan commandos, and providing assistance to the Afghan military. More than 2,000 U.S. troops have died there since the war began in 2001. Thousands of Afghan civilians have been killed as well. More than 3,000 Afghan civilians were killed in 2016 making it the deadliest year for civilians in the country since the U.N. mission there began tracking casualty numbers in 2009.

On Saturday, three U.S. soldiers were killed in an apparent insider attack in Afghanistan's restive east where U.S. Special Operations forces are battling the Islamic State's Afghan affiliate.