A US student detained in North Korea has issued a dramatic and emotional apology, confessing in front of cameras his "severe crime", according to state media.

Otto Warmbier, 21, made his first appearance since his arrest in January in an official broadcast where he effusively apologised for his actions, saying he was encouraged by a church in Ohio and the CIA to commit the crime.

Warmbier's parents said they were "greatly relieved" they had seen pictures of him.

"We had not heard from him during these many weeks, so you can imagine how deeply worried we were and what a traumatic experience this has been for us," Fred and Cindy Warmbier said in a statement released on Monday. "He seems to be in good health, although we won't know for sure about his condition until we have a chance to speak with him."

North Korea has a long history of detaining foreigners and has used jailed US citizens in the past to extract high-profile visits from the United States, with which it has no formal diplomatic relations.

"I committed the crime of taking out a political slogan from the staff-only area of the Yanggakdo International Hotel," the North's KCNA news agency quoted Warmbier as telling media in Pyongyang, the North Korean capital. It did not say what the item was.

CNN showed video of a sobbing Warmbier saying: "I have made the worst mistake of my life, but please act to save me."

Warmbier said a "deaconess" had offered him a used car worth $10,000 if he could present a US church with the slogan as a "trophy" from North Korea, KCNA said.

The acquaintance also said the Friendship United Methodist Church in Ohio would pay his mother $200,000 if he was detained by the North and did not return, KCNA quoted Warmbier as saying.

The church's senior pastor has denied knowledge of the deaconess named by North Korean officials, and said Warmbier is not a member of the church.

"My crime is very severe and pre-planned," Warmbier was quoted as saying, adding that he was impressed by North Korea's "humanitarian treatment of severe criminals like myself."

The White House said on Monday it was aware of Warmbier's situation and was working closely with Sweden, the United States' protecting power in North Korea, to learn as much as it can about his detention.

Warmbier "was caught committing a hostile act against the state", according to North Korea's state media in January which added the act was "tolerated and manipulated by the US government".

Although most tourists to North Korea are from China, roughly 6,000 Westerners visit annually, despite strong recommendations against travel to the totalitarian state from the United States and Canada.

Most are curious about life behind the last sliver of the iron curtain and ignore critics who say their dollars prop up a repressive regime.

The oppressive communist state is considered to be the worst place to live as Christian with 70,000 estimated to be imprisoned in labour camps for their faith.

Additional reporting from Reuters.