Sen. John McCain said Thursday he has full confidence in Gen. John "Mick" Nicholson, the U.S. commander in Afghanistan, in the wake of reports that President Trump may fire the general.

The U.S. failure in the Afghanistan campaign is due to years of White House policies that have continued under Trump, and is not due to military leaders, said McCain, who is in Arizona for brain cancer treatment.

"Gen. John Nicholson has served our country with honor and distinction for 35 years. He has earned the trust and admiration of those he has served with. And he has earned my full confidence," the Arizona Republican and chairman of the Armed Services Committee said in a statement.

Trump lashed out at Defense Secretary Jim Mattis and other military leaders and considered firing Nicholson during a tense July meeting aimed at producing a new strategy in Afghanistan, according to exclusive reporting by NBC News.

The president left the meeting without deciding on a new strategy, the network reported.

"I urge the president to resolve the differences within his administration as soon as possible and decide on a policy and strategy that can achieve our national security interests in Afghanistan and the region," McCain said.

This year, Nicholson testified to Congress that a few thousand troops are needed to stabilize the country amid a resurgent Taliban and emergence of the Islamic State.

Mattis, Joint Chiefs Chairman Gen. Joseph Dunford and Lt. Gen. H.R. McMaster, Trump's national security adviser, have been working for months to write a new strategy that could include 3,000-4,000 additional troops, though the president has reportedly been dissatisfied with his options for the war-torn country, where two previous administrations have struggled to achieve security.

McCain repeated his threat to put his own Afghanistan security plan into the annual defense policy bill when he and the Senate return to Washington in September for the fall session.