Washington said it would "engage in all possible channels" to win the release of Laura Ling and Euna Lee, reporters for former Vice President Al Gore's San Francisco-based Current TV media venture.
There are fears Pyongyang is using the women as bargaining chips as the U.N. debates a new resolution to punish the country for its defiant May 25 atomic test and as North Korea seeks to draw Washington into direct negotiations.
The journalists were found guilty of committing a "grave crime" against North Korea and of illegally entering the country, state-run media said.
The Central Court in Pyongyang sentenced each to 12 years of "reform through labor" in a North Korean prison after a five-day trial, the Korean Central News Agency said in a terse, two-line report that provided no further details. A Korean-language version said they were convicted of "hostility toward the Korean people."
The ruling, nearly three months after their arrest, comes amid soaring tensions fueled by North Korea's nuclear test last month and signs it is preparing for a long-range missile test. On Monday, North Korea warned fishing boats to stay away from the east coast, Japan's coast guard said, raising concerns more missile tests are being planned.
Over the weekend, President Barack Obama used strong language on North Korea's nuclear stance and said his administration did not intend "to continue a policy of rewarding provocation."
Verdicts issued by North Korea's highest court are final and cannot be appealed, said Choi Eun-suk, a North Korean law expert at the Institute for Far Eastern Studies at South Korea's Kyungnam University. He said North Korea's penal code calls for transferring them to prison within 10 days.
The United States, which does not have diplomatic ties with Pyongyang, was "deeply concerned" about the reported verdict, State Department spokesman Ian Kelly said in Washington. He said officials would "engage in all possible channels" to win the reporters' release
The families of Lee, 36, and Ling, 32 — sister of National Geographic "Explorer" TV journalist Lisa Ling, who pressed publicly for their release last week — had no immediate comment, spokeswoman Alanna Zahn said from New York. Gore also had no comment, spokeswoman Kalee Kreider said.
Kim Yong-hyun, a professor at Seoul's Dongguk University, said the 12-year sentence — the maximum allowed under North Korean law — may have been a reaction to recent "hard-line" threats by the U.S., including possible sanctions and putting North Korea back on a list of state sponsors of terrorism.
But he predicted the journalists' eventual release following diplomatic negotiations.
"The sentence doesn't mean much because the issue will be resolved diplomatically in the end," Kim said.
North Korean guards arrested Ling and Lee near the China-North Korean border on March 17. The two were reporting about the trafficking of North Korean women at the time of their arrest, and it's unclear if they strayed into the North or were grabbed by aggressive border guards who crossed into China. A cameraman and their local guide escaped.
Just weeks later, North Korea launched a multistage rocket over Japan in defiance of international calls for restraint. The U.S. and others called the launch a cover for a long-range missile test, and the U.N. Security Council condemned the move.
The U.N. censure enraged Pyongyang. North Korea abandoned nuclear disarmament talks, threatened to restart its atomic program and vowed to conduct nuclear and long-range missile tests if the Security Council failed to apologize.
The North followed through with its threat and staged its second-ever underground nuclear test. U.S. officials say the North appears to be preparing another long-range missile test at a west coast launch pad.
Some analysts called the arrest of the Americans a timely "bonanza" for Pyongyang as the impoverished regime prepares to negotiate for aid and other concessions to resolve the tense standoff over its nuclear defiance.
"North Korea refused to release them ahead of a court ruling because such a move could be seen as capitulating to the United States," said Hajime Izumi, professor of international relations and an expert on North Korea at the University of Shizuoka in Japan.
But now, "North Korea may release them on humanitarian grounds and demand the U.S. provide humanitarian aid in return," he said. "North Korea will certainly use the reporters as a bargaining chip in negotiations with the United States."
Their release could come through a post-negotiation political pardon, said Yang Moo-jin, a professor at Seoul's University of North Korean Studies.
Lisa Ling, who in a blog entry described "the feeling of utter isolation" she experienced during a 2005 trip to North Korea, had pleaded for leniency. She said neither journalist intended to cross into North Korea and her younger sister suffered from an ulcer requiring medical treatment, while Lee was the mother of a 4-year-old, Hannah.
The sentence is "a terrible shock for all those who have repeatedly insisted on their innocence," Reporters Without Borders said in a statement, noting that North Korea is ranked as Asia's worst country for press freedom.
It comes a month after Iran released Iranian-American journalist Roxana Saberi, who had been sentenced to eight years in prison for on a charge of spying for the United States. An appeals court reduced that to a two-year suspended sentence and she was freed May 11.
Another American who stood trial in North Korea in 1996 was treated more leniently. Evan C. Hunziker, apparently acting on a drunken dare, swam across the Yalu River dividing North Korea from China.
He was accused of spying and detained for three months before being freed after New Mexico Gov. Bill Richardson, then a congressman, traveled to Pyongyang to negotiate for his release.
The North Koreans wanted Hunziker to pay a $100,000 criminal fine but eventually agreed on a $5,000 payment to settle a bill for a hotel where he was detained.