Sen. John McCain was back in Arizona Monday to begin cancer treatment but it did not stop him from again hammering the Trump administration over its lack of an Afghanistan strategy.

The Republican chairman of the Armed Services Committee, one of the loudest critical voices on Capitol Hill before his cancer diagnosis, issued a new warning for the administration to finish up its overdue plan for the war-torn country.

Otherwise, McCain said he will still make good on a threat to force a strategy on the president as part of the Senate's annual defense policy bill.

"More than six months after President Trump's inauguration, there still is no strategy for success in Afghanistan," he said in a released statement. "When the Senate takes up the National Defense Authorization Act in September, I will offer an amendment based on the advice of some our best military leaders that will provide a strategy for success in achieving America's national interests in Afghanistan."

In recent weeks, McCain demanded a completed plan from Defense Secretary Jim Mattis and Trump Pentagon nominees during public Armed Services hearings, saying the U.S. is losing in Afghanistan.

"I believe in mid-July we will be able to brief you in detail, sir," Mattis said during a hearing in June. "We are putting it together now."

Yet no finished strategy has materialized and the administration is still debating a plan that could send up to 5,000 more American troops to Afghanistan, where the U.S. has been fighting the Taliban since 2001.

McCain has aggressively backed an increase in the 8,400 troops now deployed to Afghanistan as part of a new strategy. But it was uncertain what role he would play in the debate after undergoing surgery earlier this month and being diagnosed with an aggressive form of brain cancer. He left Washington last week and began targeted radiation and chemotherapy treatment today in Arizona, according to his office.

The statement on Monday was an indication McCain plans to keep up the pressure despite his dire medical issues.

The threats are a way to keep the Afghanistan issue visible and are an attempt to "shame the administration" into action on a strategy, said Thomas Spoehr, the director of the Center for National Security at the Heritage Foundation.

But McCain's threats of the Senate creating a new plan for success and then adding it to the 2018 NDAA, a massive piece of legislation that sets military policy, might be more like political bluster than a real possibility, Spoehr said.

"Sen. McCain was in the military, he knows how ludicrous it would be for Congress to come up with a military strategy," he said.