WASHINGTON, Aug 31 (AFP) - President Joe Biden made a forceful defence on Tuesday (Aug 31) of his "wise" decision to leave Afghanistan, telling Americans he refuses to send another generation to fight in a "forever war".

The traumatic departure from Afghanistan, completed on Monday after 20 years of war against the Taliban, was "a wise decision and the best decision for America," Biden said in an address to the nation.

After coming under fire from Republican opponents over the chaotic nature of the rush to the exits in Afghanistan, Biden said he did what should have been done years ago.

"I was not going to extend this forever war and I was not extending a forever exit," he said.

The evacuation, he said, was an "extraordinary success".

Speaking in the ornate State Dining Room of the White House, Biden thumped the lectern as he detailed the extraordinary costs of a war - more than 2,400 US military deaths and up to US$2.3 trillion spent - that ended with the Taliban guerrillas back in power.

"I take responsibility for the decision," he said.

"I made a commitment to the American people that I would end this war. Today, I honoured that commitment. It was time to be honest," he said.

"After 20 years in Afghanistan I refused to send another generation of America's sons and daughters to fight a war."