WASHINGTON (Reuters) - Former President Jimmy Carter plans to visit North Korea, the State Department said Thursday, but it said he was going in a private capacity and not carrying a message from the U.S. government.

Carter, a Democrat like U.S. President Barack Obama, visited the secretive, impoverished country in August and secured the freedom of a U.S. citizen who had been jailed after illegally crossing into North Korea from China.

The Carter Center in Atlanta said it would make no comment on the State Department announcement, which gave no date for the trip.

The United States, which does not have formal diplomatic relations with North Korea, suspects Pyongyang of seeking to develop nuclear weapons, a capability that could threaten U.S. allies Japan and South Korea.

While Carter has at times played a diplomatic role since he left the White House in 1981, the State Department stressed that he was not on an official U.S. government mission to the North, which has twice conducted nuclear tests.

"We have been made aware of his trip. I am not aware of any plans that we have to talk with him," State Department spokesman Mark Toner told reporters.

"He is traveling in a private capacity," Toner added. "He is not traveling with an official U.S. delegation and he does not carry an official message."

Carter visited the North in August to secure the release of Aijalon Mahli Gomes, sentenced to eight years hard labor earlier in 2010 for illegally entering the isolated state.

(Editing by David Storey)