Sen. John McCain, R-Ariz., Thursday sharply criticized President Donald Trump's appearance this week at the North Atlantic Treaty Organization summit, calling it "disappointing, yet ultimately unsurprising."

McCain, who is chairman of the Senate Armed Services Committee, also urged the president to take a tough stance in an upcoming meeting with Russian President Vladimir Putin.

Trump stunned America’s European allies with a caustic warning that he would "go it alone" unless NATO members sharply boosted defense spending. After an emergency meeting, Trump claimed a personal victory at having pressured allies to make big increases in defense outlays. French President Emmanuel Macron, however, rejected Trump's claim that NATO agreed to spending increases.

McCain, a frequent critic of the Republican president, also blasted Trump for his criticism of Germany. Trump had accused Germany of being "captive" to Russia, citing a deal to build a natural gas pipeline.

"There is little use in parsing the president’s misstatements and bluster, except to say that they are the words of one man," McCain said in a statement. "Americans, and their Congress, still believe in the transatlantic alliance and [NATO], and it is clear that our allies still believe in us as well."

After the contentious meeting with the leaders of NATO countries, Trump declared his continued commitment to the Western alliance that was formed after World War II.

“I let them know that I was extremely unhappy,” the president said, but added that the meeting ended on good terms: “It all came together at the end. It was a little tough for a little while.”

Following the NATO summit, Trump traveled to the United Kingdom and is scheduled to meet Putin on Monday in Helsinki.

McCain, who is away from the Senate as he battles brain cancer, also said Trump’s meeting with Putin will be closely watched around the world. He urged Trump to be "strong and tough" with Putin, adding that Trump must "reverse his disturbing tendency to show America’s adversaries the deference and esteem that should be reserved for our closest allies.”

“Putin is America’s enemy,” McCain said, citing Russia’s invasion of the Ukraine and its efforts “to attack America’s election and to undermine democratic institutions throughout the world."

The senator, himself a former presidential candidate, put it to Trump to stand up to the Russian leader.

"It is up to President Trump to hold Putin accountable for his actions during the meeting in Helsinki," McCain said. "Failure to do so would be a serious indictment of his stewardship of American leadership in the world."