Mr Clinton's trip to North Korea was not announced beforehand by either Pyongyang or Washington.
Officials later made clear that his aim was to secure the freedom of two US reporters, Laura Ling and Euna Lee.
Mr Clinton held talks with Kim Jong-il and his staff for more than three hours.
Senior US sources said North Korea had indicated in advance that it would free the journalists if Mr Clinton made a private visit. During the trip Mr Kim granted the reporters an official pardon.
The women had been sentenced to 12 years' hard labour after being convicted of entering North Korea illegally.
They are now flying back to the US, where they will be reunited with their families in Los Angeles.
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