The airport reopened at 1935 GMT Monday, said Major General Hank Taylor, a logistics specialist on the Pentagon's Joint Chiefs of Staff.

He said a C-17 transport aircraft had landed with US Marines aboard, and a second one loaded with an army unit was to land soon, to help establish security for the airport.

Taylor said the United States was "in charge of air traffic control" at Hamid Karzai International Airport (HKIA) for military and commercial flights.

He added that there were already about 2,500 US troops in Kabul to help organize the evacuation of thousands of Americans and Afghans who worked for them as translators and in other jobs.

By the end of Monday, US time, he said there could be 3,000 to 3,500 on the ground.

"Our focus right now is to maintain security at HKIA, to continue to expedite flight operations while safeguarding Americans and Afghan civilians," he said.

The airport was shut down Monday after crowds of civilians surged onto the runways, the Pentagon said.

Videos taken from the airport showed hundreds of Afghans flooding onto the runways and trying to impede the takeoff of one of the US transports.

Meanwhile two armed Afghan men were killed by US forces as thousands sought to board flights.