According to news reports at the Chinese-North Korean border, the pair, with an activist who'd worked with them, were seized, though it's not clear if it happened in North Korean or Chinese territory.
"We don't have anything special to say about the report, and it is not for us to confirm," said Moon Tae-young, a South Korean Foreign Ministry spokesman, through a translator.
The arrests come at a time of heightened tension on the Korean Peninsula, with North Korea declaring it will shoot a satellite into space next month. Observers say that's a cover for the test-fire of a long-range missile.
The U.S. State Department is working with the Chinese government to find the Americans, because the U.S. does not have direct relations with communist North Korea. Swedish diplomats in Pyongyang are acting as intermediaries.
The Reverend Chun Ki-won of the Seoul-based Durihana Mission, a Christian group that aids defectors, said he arranged interviews with North Korean defectors, but warned the Americans to stay aware of the border.
The Tumen and Yalu rivers dividing North Korea and China are frequent crossing points for trade, and the growing number of North Koreans fleeing their country. In North Korea's northeast, the Tumen is frozen over in places, making crossing easier.
Laura Ling was sending updates about her trip through the online site Twitter. A week ago she wrote: "Spent the day interviewing young North Koreans who escaped their country ... too many sad stories. "Her most recent entry was Monday. It said simply: "Missing home."
The location of the American detainees is unknown.
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