Intelligence agencies in Japan, South Korea and the US believe the rocket launch is cover for a test of a long-range missile.
The two journalists seized are believed to Euna Lee, a Korean American, and Laura Ling, a Chinese American, both of whom are working for California-based Current TV, an online media outlet set up by Al Gore, the former US vice-president.
They are thought to have been taken along with a Chinese guide who was helping them.
A third journalist in the group is thought to have escaped capture.
Speaking to the AFP news agency, a spokesman for the state department in Washington said the two were taken into custody on Tuesday "across the Tumen river by what appear to be North Korean border guards".
"We are working with Chinese government officials in that particular area to ascertain the whereabouts and welfare of the Americans in question," Fred Lash, a press officer, said.
"We've also been in touch with North Korean officials to express our concern about the situation."
The Tumen river in the east and the Yalu river in the west form most of the border between China and North Korea and is a common escape route for refugees fleeing the North.
|The US has no diplomatic relations with the North Korean government [GALLO/GETTY]
Chun Kiwon, a human rights activist and Protestant pastor who said he helped arrange the reporters' visit to China, told AFP that they had been held after they "apparently got too close to the North Korean side of the Tumen river".
"They told me they were going to do a programme on North Koreans who have fled the North," said Chun, who heads a missionary group providing assistance to North Korean defectors.
North Korea has detained American citizens on previous occasions after accusing them of infringing its borders.
In 1996, Bill Richardson, then a US congressman, travelled to Pyongyang to negotiate the release of Evan Hunziker, who had been held for three months after a drunken swim across the Yalu river.
North Korea had accused Hunziker of being a spy.
Two years earlier Richardson also helped negotiate the release of a US military pilot shot down after his helicopter strayed into North Korean airspace.
The US has no diplomatic relations with North Korea and channels of communication between Washington and Pyongyang are limited.
The Swedish embassy which represents US interests in the North Korean capital has made no comment on the case of the two journalists.
Officials at the Chinese foreign ministry in Beijing told reporters on Thursday that they were investigating the case but did not elaborate.