The president also said the evacuation of Americans, at-risk Afghans and others from Kabul airport accelerated over the weekend - and claimed the area remains vulnerable to threats posed by the Islamic State extremist group.
Speaking from the White House on Sunday, President Biden insisted now was the right time to remove troops from the country.
He said: "Let me be clear, the evacuation of thousands of people from Kabul is going to be hard and painful no matter when it started or when we began.
"It would have been true if we started a month ago or a month from now.
"There is no way to evacuate this many people without pain or loss and the heartbreaking images you see on television, it is just a fact."
President Joe Biden says the evacuation of people from Kabul would be 'painful' no matter when
Biden and his top aides have repeatedly cited their concern that extremist groups such as Isis will attempt to exploit the chaos around the Kabul airport.
Jake Sullivan, the national security adviser, told CNN the threat from Isis was “real, acute, persistent and something we’re focused on with every tool in our arsenal”.
One week after the Taliban completed its takeover of Afghanistan by capturing Kabul, Biden said discussions are also underway among military officials about potentially extending the airlift beyond Biden's August 31 deadline.
“Our hope is we will not have to extend, but there are discussions,” he said, suggesting the possibility that the Taliban will be consulted.
Since August 14, one day before the Taliban entered Kabul, the airlift has evacuated 28,000 people, Biden said.
He said that included 11,000 who had departed from Kabul in a 36-hour period this weekend, but he did not provide details.
The number appeared to include flights by charter and non-U.S. military aircraft as well as the U.S. Air Force C-17 and C-130 transport planes that have been flying daily from the capital.
Tens of thousands of people remain to join the airlift, which has been slowed by security issues and U.S. bureaucracy hurdles.
Biden asserted, without a full explanation, that U.S. forces have managed to improve access to the airport for Americans and others seeking to get on flights. He suggested that the perimeter had been extended, widening a “safe zone.”
“What I’m not going to do is talk about the tactical changes we’re making to make sure we maintain as much security as we can," he said.
"We have constantly, how can I say it, increased rational access to the airport, where more folk can get there more safely. It's still a dangerous operation but I don’t want to go into the detail of how we’re doing that.”
Later Biden added: “We've discussed a lot with the Taliban. They’ve been cooperative in extending some of the perimeter.”
He said groups of Americans in Kabul are being moved more efficiently and safely to the airport, but he provided no details.
“Any American who wants to get home, will get home,” he asserted.
Addressing a criticism cited by many Republicans, Biden said no Afghan evacuees are being flown directly to the United States from Afghanistan without prior screening.
He said they are being screened in third countries.
The Biden administration has given no firm estimate of the number of Americans seeking to leave Afghanistan. Some have put the total between 10,000 and 15.000.
The British military said Sunday another seven people had been killed in the unceasing crush of crowds outside the airport.
Prime Minister Boris Johnson said he would convene leaders of G7 countries Canada, France, Germany, Italy, Japan, and the United States on Tuesday for "urgent talks" on the situation in Afghanistan.
He said: "It is vital that the international community works together to ensure safe evacuations, prevent a humanitarian crisis and support the Afghan people to secure the gains of the last 20 years."