Two powerful explosions outside Kabul airport killed as many as 20 people on Thursday, just hours after Western nations warned of an imminent terror threat and thousands of people gathered hoping for a flight out of Taliban-controlled Afghanistan.

The blasts came as the August 31 deadline looms for the United States to withdraw its troops, and for it and other Western countries to end a massive airlift that has already evacuated nearly 100,000 people.

The airport is the only part of the country under foreign control following the Taliban's return to power on August 15, and huge crowds have massed in the hope of being evacuated.

What we know so far

• Two explosions went off outside the Kabul airport.
• At least 13 people have been killed and more than 50 injured.
• US officials say one of the explosions appears to be a suicide bomb.

Zabihullah Mujahid, the Taliban's main spokesman, told AFP that “between 13 and 20” people were killed and 52 wounded in the twin blasts, while Kabul hospitals reported six dead and up to 90 wounded.

The Pentagon said “a number” of US troops died in what spokesman John Kirby called a complex attack.

US President Joe Biden had earlier cited an “acute” terrorist threat from the regional chapter of the militant Islamic State group.

The Taliban condemned the blasts, saying they were in an area under US military control.

“The Islamic Emirate strongly condemns the bombing targeting civilians at Kabul airport,” said a statement released by Mujahid on Twitter.

Thousands of people have been gathering outside the airport in recent days. Western troops are racing to evacuate foreigners and Afghans who helped Western countries during the 20-year war against the Taliban, and to get out themselves by an Aug 31 deadline.

Pentagon spokesman John Kirby said one blast occurred near the airport's Abbey Gate and the other close to the nearby Baron Hotel. Two US officials said at least one of the explosions appeared to be from a suicide bombing.

“We can confirm that the explosion at the Abbey Gate was the result of a complex attack that resulted in a number of US & civilian casualties,” Kirby said on Twitter. “We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate.”

A US official told Reuters as many as three US service members were among those hurt, and that US casualty numbers were expected to increase, according to initial information. At least one of the US personnel was seriously hurt, an official said.

The US Embassy in Kabul described “a large explosion” and said there had been reports of gunfire.

US and allied officials have said they had intelligence that suicide bombers were threatening to attack the airport.

US President Joe Biden has been briefed on the explosion, according to a White House official.

Biden was in a meeting with security officials about the situation in Afghanistan when the explosion was first reported, according to a person familiar with the matter.

Britain's defence ministry said it was working urgently to establish what had happened at Kabul airport following reports of an explosion.

“We are working urgently to establish what has happened in Kabul and its impact on the ongoing evacuation effort,” the defence ministry said on Twitter.

“Our primary concern remains the safety of our personnel, British citizens and the citizens of Afghanistan. We are in close contact with our US and other Nato allies at an operational level on the immediate response to this incident.”

Western nations warn citizens at airport

Earlier today, Western nations warned their citizens to immediately leave the surrounds of Kabul airport over a terrorist threat, as thousands of people try to reach a dwindling number of evacuation flights.

The US government and its allies raised the alarm with a series of coordinated and specific advisories for their citizens to avoid the airport.

“Those at the Abbey Gate, East Gate, or North Gate now should leave immediately,” said the US State Department, citing unspecified “security threats”.

Australia's department of foreign affairs said there was an “ongoing and very high threat of terrorist attack”. “Do not travel to Kabul Hamid Karzai International Airport. If you're in the area of the airport, move to a safe location and await further advice.”

London issued a similar warning, adding “if you can leave Afghanistan safely by other means, you should do so immediately.”