The military airlift of thousands of people fleeing the Taliban takeover of Afghanistan is posing a massive challenge to international forces, heavily dependent on securing Kabul airport and access from surrounding areas, experts say.
Dozens of aircraft from around 15 countries are scrambling to evacuate tens of thousands of foreign and Afghan nationals from Kabul's Hamid Karzai Airport since the Taliban swept back into power on Sunday.
In doing so, they are relying on contingency plans drawn up from experience of previous disasters, such as the devastating Haiti earthquake in 2010, according to Ben Carves, an analyst at Rand Europe.
"You'll have specific contingency plans for locations that have been identified as being high risk and Kabul will probably be one of those," said Carves, a former British Royal Air Force logistics officer.
"The challenge comes when you have to coordinate that multinationally, when you've got every nation trying to do the same thing, how do you stop that from becoming complete chaos?" said Carves.
With the sudden collapse of the Afghan government, the US military, which plans to evacuate more than 30,000 people by August 31, has taken over air traffic control and flight coordination at the airport.
According to the daily US Air Force Times, control of air movements into Kabul is the responsibility of US airmen based at Al Udeid Air Base in Qatar. The Afghan Civil Aviation Authority said Monday it could no longer control civilian traffic over Afghanistan.
Washington is now counting on one plane "arriving and leaving Kabul per hour" and the "departure of 5,000-9,000 passengers per day", US General Hank Taylor, Deputy Director of the US Joint Chiefs of Staff, told a press conference.
- Security concern -
Some of the 4,000 US troops in Kabul -- soon to be 6,000 -- are also tasked with for managing aircraft movements on the ground, said John Wesley, a military airlift specialist at MilAirX.
"When it comes to this massive operation, it's not the size I'm concerned about," said Wesley.
"My concern is the security, and how fast our troops have had to react. It's always safer to control the situation, rather than react to the situation. Our troops are being forced to make decisions right there on the spot that are far from ideal."
The huge Baghram air base north of Kabul would have been easier to secure if it had not been abandoned by the United States in early July, according to Wesley.
The invasion of Kabul airport's runways on Monday by a crowd of anguished and panicked Afghans caused chaos and led to a temporary halt in all air traffic.
"The fact that they have re-instigated flights again suggests they've got that under control now," said Carves.
In addition to the Americans, Turkey has deployed 900 troops to help secure the airport. Britain has also sent 900 troops and France an unspecified number of special forces.
They must ensure that only authorised people board the planes. "Because in a crisis like this, anyone can try to board a flight. In the worst case, a terrorist," warned Carves.