President Joe Biden said he stands "squarely behind" his decision to withdraw US forces from Afghanistan, and that the government's collapse was quicker than anticipated.

Biden said he was faced with a choice between sticking to a previously negotiated agreement to withdraw US troops this year or sending thousands more service members back into Afghanistan for a "third decade" of war.

Biden said he will not repeat mistakes of the past and did not regret his decision to proceed with the withdrawal.

"I stand squarely behind my decision," Biden said in a televised address to the nation from the White House East Room.

"After 20 years, I've learned the hard way that there was never a good time to withdraw US forces."

Biden said he would rather take the criticism over the fallout in Afghanistan than leave the decision to another president.

He said the decision to leave Afghanistan is "the right one for America."