The arrest put the number of Americans held in North Korea at four, as Washington and Pyongyang remain at loggerheads over the North's nuclear and missile ambitions.

While a travel warning already was in effect for US nationals, the State Department offered a more detailed statement about the pitfalls of heading to North Korea.

"The Department of State strongly warns US citizens not to travel to North Korea," the warning said. "US citizens in the DPRK are at serious risk of arrest and long-term detention under North Korea's system of law enforcement," the department said.

"This system imposes unduly harsh sentences for actions that would not be considered crimes in the United States," it added, reminding Americans that the two countries do not have diplomatic relations. 

Pyongyang has carried out two nuclear tests and dozens of missile launches since the beginning of last year in its quest to build a rocket capable of delivering an atomic warhead to the US mainland. 

Washington has suggested military action could be on the table but President Donald Trump has softened his message more recently, saying he would be "honored" to meet North Korean leader Kim Jong-Un. (AFP)