John Kerry, who knows something about having his war record attacked, savages Donald Trump

Secretary of State John Kerry came to the defense late Saturday night of Sen. John McCain (R-Arizona), whose combat record had come under fire earlier in the day by Republican presidential candidate Donald Trump.

Kerry served in the US Senate with McCain for nearly 30 years before replacing Hillary Clinton as secretary of state near the start of President Barack Obama's second term.

"I have known John McCain for more than 30 years. We've had our share of disagreements and still do today. But one thing I know is beyond debate is that John McCain is a hero, a man of grit and guts and character personified," Kerry said in the statement.

"He served and bled and endured unspeakable acts of torture. His captors broke his bones, but they couldn't break his spirit, which is why he refused early release when he had the chance. That's heroism, pure and simple, and it is unimpeachable."

Kerry went on to blast Trump without mentioning his name.

"If anyone doesn't know that John McCain is a war hero, it only proves they know nothing about war and even less about heroism," Kerry said.

Earlier in the day during a forum in Iowa, the real-estate magnate Trump had questioned McCain's status as a "war hero." Trump, who has been in a war of words with McCain for much of the past week over his views on immigration, disparaged McCain for being "captured" during the Vietnam War.

"He's not a war hero," Trump said. "He's a war hero because he was captured. I like people that weren't captured."

"Perhaps he's a war hero, but right now, he's said bad things about a lot of people," Trump later added.

McCain's plane was shot down over North Vietnam in 1967. He spent five years in a prisoner-of-war camp, where he was tortured. He has been an outspoken advocate for fair treatment of prisoners of war, breaking with many of his Republican colleagues to close the detention facility at Guantanamo Bay and clashing with the Bush administration over its use of "enhanced interrogation techniques."

Republican rivals quickly moved to condemn Trump's remarks, as many have for his assertion that the Mexican government has sent "rapists" and drug runners across the border to the United States. McCain took issue with a speech Trump gave last week in Phoenix, where McCain said Trump had fired up "the crazies" in the Republican Party.

For his part, Trump has said he doesn't plan to apologize, even saying that McCain should offer an apology to those he called "crazies."

Kerry has also endured a highly public scrutiny of his war record. During the 2004 presidential election, when the Democratic nominee Kerry ran against incumbent Republican president George W. Bush, a group known as the "Swift Boat Veterans for Truth" disparaged Kerry's service and attacked the awarding of some combat medals to Kerry.

The group's allegations were widely discredited, but not before they became a key theme of the 2004 campaign.