CNN News reported that three US officials and a source familiar with the situation said that according to initial reports, there were some US personnel among the casualties.
Pentagon spokesman John Kirby posted a tweet that read, "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. We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate. We will continue to update."
According to three US officials, the blast took place at one of the entry gates of the Hamid Karzai International Airport and appears to be a suicide attack.
Scores of Afghans are seen gathering at the airport gates trying to gain access, which has become the only way out of the country since the Taliban took over Kabul.
Meanwhile, the US Embassy in Kabul said that US citizens who were at the Abbey Gate, East Gate, or North Gate of the airport "should leave immediately" in the wake of the blast.
"There has been a large explosion at the airport, and there are reports of gunfire," the security alert said. "US citizens should avoid traveling to the airport and avoid airport gates at this time," it added.
David Martinon, the French Ambassador to Afghanistan warned that a second explosion at the Kabul airport remained a possibility.
In a tweet, he said, "To all our Afghan friends, if you are close to the doors of the airport, get away as quickly as possible and shelter yourselves. A second explosion is possible."
In the past two weeks, tens of thousands of people have been evacuated by the US military and NATO allies from the airport. Ahead of the August 31 deadline for the final exit from a 20-year war in Afghanistan, these evacuations are set to start winding down in the next few days.
The top US diplomat in Kabul, Amb. Ross Wilson, on Thursday said that he could not get into the specifics of the security threat outside the gates of the Kabul airport cited in an embassy alert, but noted that "it was clearly regarded as credible, as imminent, as compelling."
Meanwhile, US President Joe Biden has been briefed on the explosion outside Kabul airport. The President was in the situation room along with Defence Secretary Lloyd Austin and Secretary of State Antony Blinken.