It is "time to end America's longest war," US President Joe Biden will say Wednesday as he announces Washington's withdrawal from Afghanistan, which it invaded in the wake of the 9/11 attacks two decades ago.

"I am now the fourth American president to preside over an American troop presence in Afghanistan... I will not pass this responsibility to a fifth," Biden will say according to prepared remarks.

The United States will continue to support the Afghan government against the Taliban, Biden will say, and "our diplomatic and humanitarian work will continue."

But America "will not stay involved in Afghanistan militarily," he will say.

The excerpts of Biden's remarks did not include a date for the withdrawal. On Tuesday officials said it would come before September 11, 2021, and would be without conditions, despite mounting fears of a return to Taliban rule.

"We cannot continue the cycle of extending or expanding our military presence in Afghanistan hoping to create the ideal conditions for our withdrawal, expecting a different result," Biden will say according to the prepared remarks.

The US invaded Afghanistan in 2001 and toppled the repressive, ultra-conservative Taliban regime, which had hosted Al-Qaeda in the run-up to the attacks on the World Trade Center and the Pentagon. 

In the nearly 20 years since, Afghans, especially women, have asserted their rights and made hard-won strides in education and democracy, though the gains remain fragile.

But the Taliban swiftly launched an bloody insurgency that a US-led international coalition and the Afghan government has failed to quell.

In February 2020 they struck a deal with the administration of Biden's predecessor Donald Trump that would have seen US troops withdraw by May 2021 in exchange for security guarantees.

With the fighting continuing, Biden has pushed that deadline back, but removed the conditions.

US intelligence warned this week that the government in Kabul "will struggle" to hold the "confident" Taliban off if the coalition withdraws.

Many Afghans -- especially women, who have been largely shut out of peace talks between the insurgents and Kabul -- have long feared a return to the Taliban's repressive Islamist regime if the US withdraws.

Analysts also fear a descent into civil war if Kabul is left to face the Taliban alone. A United Nations report issued Wednesday stated that "Extraordinary levels of harm inflicted on civilians in #Afghanistan conflict continues unabated."

"Urgent action required by all parties to stop the violence," the UN Assistance Mission in Afghanistan said in a tweet.