The statement expressed concern about the women's health, noting that Ling, 32, has a serious medical condition and that Lee's 4-year-old daughter is showing "signs of anguish over the absence of her mother".
The White House has said it is using "all possible channels" to secure the release of two US journalists jailed in North Korea.
There are fears the journalists will be used as bargaining chips as the UN debates a new resolution to punish North Korea for nuclear tests in May.
Speaking after the sentence was announced Hillary Clinton, the US secretary of state, said the women's case should be viewed as a separate humanitarian matter from the standoff over Pyongyang's nuclear weapons test.
She said she hoped North Korea would "grant clemency and deport them".
North Korea maintains a network of labour camps where human rights groups say inmates – including thousands of political prisoners - are overworked, underfed and brutality is the norm.
However North Korea watchers say it is unlikely the two reporters will be sent to the worst camps and that rather Pyongyang is trying to maximise its leverage with
Roh Jeong-ho, the director of the Centre for Korean Legal Studies at Columbia Law School, told the Associated Press he doubted it was in North Korea's interests to force them into hard labour.
By not accusing the women of espionage it has offered a "face-saving way of resolving the issue", Roh said.
"Essentially, it's a whole package of brinksmanship."
Bill Richardson, governor of the US state of New Mexico who has previously negotiated the release of Americans in North Korea, said that the Obama administration had contacted him for advice in the case and that he had also spoken to the women's families.
|Laura Ling and Euna Lee have been sentenced to 12 years hard labour [EPA]
Richardson told US television channel NBC that political negotiations would now begin in a "high stakes poker game" for their release.
The two women, who were detained by North Korean border guards on March 17 and have been held in the country ever since were sentenced following a brief trial behind closed doors, the North's official Korean Central news agency (KCNA) said.
The journalists, who were working for California-based Current TV, an online and television service set up by Al Gore, the former US vice-president, cannot appeal because they were tried in the country's highest court, where decisions are final.
North Korea has said the two reporters had crossed illegally to its side of the border with China, although other reports have suggested the two were on the Chinese side when they were arrested.
Previous reports from North Korea have said Ling and Lee were accused of "hostile acts" but did not give details.
Analysts say the sentences appeared unusually harsh and seemed to back up views that the journalists could be used as a bargaining chip by the North in its standoff with the US.
Their arrest and trial come against a background of heightened tensions on the Korean peninsula.
On Sunday Clinton said Washington was looking into putting North Korea back on a list of state sponsors of terrorism, a designation that could subject the impoverished state to more financial sanctions.
The US removed North Korea from the blacklist in October in a bid to revive faltering six-party nuclear disarmament talks, prompting the North to take some measures to disable its nuclear facilities.
But Pyongyang has since reversed those steps and said it had restarted its nuclear complex, including reprocessing nuclear fuel to obtain weapons-grade plutonium.
North Korea has also vowed to retaliate with "extreme" measures if the United Nations punished it for conducting its nuclear test last month.
The security council has been deliberating its response and may issue a new resolution as early as this week, although there appear to be division among members over how tough sanctions should be.
"Our response would be to consider sanctions against us as a declaration of war and answer it with extreme hardline measures," the North's official Rodong Sinmun newspaper said in a commentary on Monday.