WASHINGTON (AFP) - United States (US) Senator and former prisoner of war John McCain said on Monday that Vietnam's willingness to suffer "immense casualties" was the linchpin in legendary General Vo Nguyen Giap's defeat of American forces.

Hours after the news of Gen Giap's death Friday at age 102, Mr McCain in a brief tweet praised the legendary general as a "brilliant military strategist" who once called the US an honorable enemy.

But in a Wall Street Journal op-ed, Mr McCain called into question the morality of the Vietnamese tactic, which he said Gen Giap executed with an "unbending will".

"It's hard to defend the morality of the strategy. But you can't deny its success," wrote Mr McCain.

"Giap was a master of logistics, but his reputation rests on more than that," he added. "His victories were achieved by a patient strategy that he and Ho Chi Minh were convinced would succeed - an unwavering resolve to suffer immense casualties and the near total destruction of their country to defeat any adversary, no matter how powerful."

"'You will kill 10 of us, we will kill one of you,' Mr Ho told the French, 'but in the end, you will tire of it first.'"

Gen Giap's near-mythic victory over the French in 1954's siege at Dien Bien Phu ended Paris's rule in Indochina and precipitated nearly two decades of US involvement in Vietnam. The general then used similar tactics to wear down the Americans.

"The US never lost a battle against North Vietnam, but it lost the war. Countries, not just their armies, win wars," Mr McCain said.

"Giap understood that. We didn't. Americans tired of the dying and the killing before the Vietnamese did."

Mr McCain spent five and a half years as a POW, enduring torture and solitary confinement.

Perhaps because Mr McCain was son of the commander of all US forces in the Pacific, Gen Giap paid him a visit in hospital after his US Navy jet was shot down while on a bombing mission over Hanoi in 1967.

"He stayed only a few moments, staring at me, then left without saying a word," Mr McCain wrote.

The American met Gen Giap again in the early 1990s on a return trip to Hanoi as a senator seeking ways to normalise relations.

"Both of us clasped each other's shoulders as if we were reunited comrades rather than former enemies," Mr McCain said.

He wanted to discuss Gen Giap's historical role in the wars. The general answered only briefly, then brushed the queries aside saying that was all in the past, according to Mr McCain.

"We stood up, shook hands, and as I turned to leave, he grasped my arm, and said softly, 'you were an honorable enemy'."