The United States has announced the end of its 20-year-old war in Afghanistan as the last American military flight flew out of the Hamid Karzai International Airport in Kabul shortly before the August 31 deadline.

The United States has begun a new diplomatic mission in Afghanistan, Secretary of State Antony Blinken said as the US withdrew its last troops from the country ending the 20-year war in the South Asian nation.

Blinkin said the US will engage with the Taliban not on the basis of what a Taliban-led government says, but what it does to live up to its commitments.

'No other military in the world could accomplish what we and our allies and partners did in such a short span of time. That is a testament not only to our forces' capabilities and courage but also to our relationships and the capabilities of our allies and partners,' Austin said in a statement at the conclusion of the American mission in Afghanistan.

"Now, our 20-year military presence in Afghanistan has ended," he said, thanking armed forces for their execution of the dangerous retrograde from Afghanistan as scheduled -- in the early hours of Tuesday (August 31) -- with no further loss of American lives.

"The situation on the ground continues to be extremely dangerous, and the threat of terrorist attacks on the airport remains high. Our commanders informed me that an attack is highly likely in the next 24-36 hours," US President Joe Biden said.

An ISIS-Khorasan planner was killed in a "counterterrorism" operation conducted by US military forces in the Nangahar province, a day after a terror attack in Kabul killed scores of people including 13 US troops.

The United States intelligence community has failed to reach any firm conclusion on the exact origin of COVID-19 and is split on whether it leaked from a lab in China or emerged in nature, even though it does not believe the virus was developed as a biological weapon, according to results of a detailed review ordered by President Joe Biden.

The US has said it will go ahead with its plan to conclude its Afghanistan mission by August 31, even as the evacuation effort, which is now in its "retrograde period", is facing an "ongoing and acute threat" from ISIS-K.

The president said the ISIS-K was behind the gruesome attack at the Hamid Karzai International Airport in Kabul and at a hotel nearby.

The US has held IS-K, the regional affiliate of the Islamic State terror outfit, responsible for Thursday's Kabul airport attack in which 13 American soldiers were killed and 18 others injured.

The terrorist attack was carried out by ISIS gunmen, General Kenneth Franklin McKenzie, Commander of the United States Central Command, told reporters at a Pentagon news conference on Thursday, hours after the terrorists opened gunfire at American soldiers standing outside the Hamid Karzai International Airport in Kabul.

The Hamid Karzai International Airport is the only access point for the international community to reach out to people in Afghanistan, a land locked country.

"But the completion by August 31 depends on the Taliban continuing to cooperate and allow access to the airport for those who were transporting out and no disruptions to our operations," US President Joe Biden said.

The Biden Administration on Monday said it was now focused on completing its evacuation mission from Afghanistan by August 31, the deadline for removing all American troops from the country.

"We are in talks with the Taliban on a daily basis through both political and security channels. I'm not going to get into the details of those discussions here, to protect those discussions, which are covering a wide range of issues," Sullivan told reporters at a White House news conference.

Amid deepening crisis in Afghanistan, United States President Joe Biden defended his move to withdraw troops from the country, saying history will record this as a 'logical, rational, and right decision'.

The United States is going to retain a laser focus on its counterterrorism mission in Afghanistan, President Joe Biden said on Friday, and warned the Taliban of a forceful response to any attack on American forces or disruption of its operations at the Kabul airport.

Conference organisers, who are requesting anonymity, claim the three-day event starting September 10 is cosponsored by more than 45 centres or departments at over 40 universities, most of whom are from the United States.

The new travel advisory came in the wake of the significant improvement in Covid situation in India.