Sen. John McCain (R-Ariz.) at a Senate Armed Services Committee hearing in March 2017. (Tasos Katopodis/AFP/Getty Images)

Sen. John McCain issued a stern warning Thursday to President Trump in advance of his summit with Vladi­mir Putin, saying that it would be “a serious indictment” of Trump’s leadership if he does not hold the Russian leader accountable on a range of actions.

McCain (R-Ariz.) issued a statement shortly after Trump had departed a meeting of NATO leaders in Brussels — a gathering at which McCain characterized Trump’s diplomatic skills as “disappointing, yet ultimately unsurprising.”

Trump is scheduled to meet with Putin next week in Helsinki in what Trump characterized Thursday as a “loose meeting,” adding that “we’ll see where it leads.”

Speaking at a news conference before leaving Brussels, Trump said he would “of course” raise the issue of Russian interference in the 2016 U.S. election but continued to strike a friendly tone toward Putin, saying he considered him more a “competitor” than an enemy.

McCain, who has been at home in Arizona battling brain cancer, took issue with that characterization.

“Putin is not America’s friend, nor merely a competitor,” McCain said in the statement. “Putin is America’s enemy — not because we wish it so, but because he has chosen to be.”

McCain then ticked off a series by transgressions by Putin, including the “attack” on the U.S. election and efforts “to undermine democratic institutions throughout the world.” In addition, McCain cited the Russian annexation of Crimea and his support of Syrian President Bashar al-Assad’s “slaughter” of the Syrian people.

“The president’s task is to reverse his disturbing tendency to show America’s adversaries the deference and esteem that should be reserved for our closest allies,” McCain wrote. “. . . It is up to President Trump to hold Putin accountable for his actions during the meeting in Helsinki. Failure to do so would be a serious indictment of his stewardship of American leadership in the world.”

McCain also sought to reassure NATO allies who had been rankled by Trump’s brusque approach at the two-day summit.

“President Trump’s performance at the NATO summit in Brussels was disappointing, yet ultimately unsurprising,” McCain said. “There is little use in parsing the president’s misstatements and bluster, except to say that they are the words of one man. Americans, and their Congress, still believe in the transatlantic alliance and the North Atlantic Treaty Organization, and it is clear that our allies still believe in us as well.”