(Reuters) - More than 600 Afghan men, women and children, crouching and crammed up against each other on the floor of a U.S. military plane, left Kabul after the city was seized by Taliban insurgents.

       A photograph showing the Afghan civilians - some clutching luggage, others bottle-feeding infants - on the C-17 cargo aircraft has gone viral on social media.

       A U.S. official told Reuters about 640 people clambered onto the flight from Kabul on Sunday, when thousands of people desperate to flee the country surged to the airport in the Afghan capital.

       "The unusually high number of passengers was the result of a dynamic security environment that necessitated quick decision making by the crew which ultimately ensured these passengers were quickly taken outside the country," the official said.

       According to the manufacturer, Boeing, the C-17 Globemaster III can carry 134 passengers, including 54 on side seats and 80 on pallets on the floor.

       Many of the Afghans climbed aboard the plane via a half-open ramp, before the flight left for Qatar with one of the highest number of passengers ever flown on such an aircraft, U.S. defence and security news site Defense One reported. Reuters could not immediately verify those details.

       Other distressing videos and photos have emerged from Kabul airport, where witnesses say several people have died, and show people clambering up overhead gangways and clinging to the landing gear of taxiing planes in desperate attempts to flee.

       For some observers, the image inside the C-17 plane was seen as a sign of hope and the bravery of the evacuation crew.

       "For all the failings of this week, some unalloyed good in this," said Blake Herzinger, a Singapore-based security analyst, who shared the image on Twitter.

       But for others, it served as a reminder of the calamitous evacuation efforts from Afghanistan after the United States withdrew its forces after 20 years of war and the Taliban swept to power in days rather than the months predicted by U.S. intelligence.

       "We need many more such planes," said Kenneth Roth, executive director of non-governmental organisation Human Rights Watch.

       (Reporting by John Geddie; Editing by Raju Gopalakrishnan and Janet Lawrence)