The U.S. Embassy in Kabul advised U.S. citizens not to travel to the Hamid Karzai International Airport, also known as Kabul international airport, at this time, according to an embassy security alert.

Citizens already at the airport's Abbey Gate, East Gate and North Gate were advised to leave immediately, said the security alert.

"Because of security threats outside the gates of Kabul airport, we are advising U.S. citizens to avoid traveling to the airport and to avoid airport gates at this time unless you receive individual instructions from a U.S. government representative to do so," the embassy said. 

In a similar advisory, Britain told people in the airport area to "move away to a safe location." 

"There is an ongoing and high threat of terrorist attack," the British Foreign Office said in its statement.

(With input from Reuters)