"General 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," McCain said in a statement.
"Our commanders-in-chief, not our commanders in the field, are responsible for this failure," he added, referring to Trump and former President Barack Obama. "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."
The White House and Pentagon are pushing back against an NBC News report
that Trump is weighing firing Nicholson.
In an appearance on Fox News Thursday, Trump's deputy assistant, Sebastian Gorka, said the President is confident in Nicholson's military leadership in Afghanistan.
"Absolutely, absolutely, yes," he said. "It is not a question of confidence, it is a question of inheriting bad ideas, false assumptions and reassessing what's good for America."
The Pentagon also tried to downplay reports that Nicholson could be on the way out.
Defense Secretary James Mattis "has confidence in General Nicholson's leadership," according to a statement from Pentagon spokesperson Dana W. White.
Nicholson took command of US forces in Afghanistan in March 2016 and CNN has not independently verified the NBC News report that Trump is considering removing him from his post.
But while Trump retains full confidence in his military commanders in Afghanistan, he remains skeptical about a continued US presence in the country, Gorka said on Thursday, describing a meeting last week where Trump questioned plans for the future of US troops in Afghanistan.
"Look, nothing is carved in stone," said Gorka, a national security adviser to the President.
"What the President did here in the West Wing a week ago in the situation room is he asked them very pertinent questions and he basically doesn't want this administration to make the same mistakes that both the Bush administration and the Obama administration made."
"He wants everyone to look at the core assumptions upon which our plans are based, and say are these assumptions sound," Gorka said. "The key question, Bill (Hemmer), is what is the national security relevance of Afghanistan to this country? When that question is answered adequately then we'll know which options we should apply and the President will make the decision."
The Trump administration is currently considering its commitment to and strategy for Afghanistan and the wider region.
Mattis told Congress that the strategy would be decided upon by mid-July but the plan has been delayed for months amid sharp disagreements between national security adviser H.R. McMaster, who is arguing for an increase of several thousand troops to help turn the tide in the fight against the Taliban, and the President's chief strategist Steve Bannon, who is opposed to getting the US more deeply involved in the conflict.
In the absence of an administration policy on Afghanistan, McCain, the Republican chairman of the Armed Services Committee, said Monday he would work to produce a strategy
for winning the conflict in September.
"Eight years of a 'don't lose' strategy has cost us lives and treasure in Afghanistan. Our troops deserve better," McCain said in a statement.
US and coalition casualties in Afghanistan have become rarer in recent years, falling dramatically since the Afghan government assumed responsibility for combat operations in 2014. But there has been an uptick in recent months as US forces have become more directly involved in the fight against the local ISIS affiliate.