North Korea routinely delays or denies consular access to US citizens detained in the communist nation, the State Department said Thursday, after Pyongyang threatened to "mercilessly punish" the Americans it's accusing of unspecified "crimes" against the country.

The North's Foreign Ministry made the threat earlier Thursday, claiming it is the "legitimate right of a sovereign state to deal with the criminals according to its law" and rejecting criticism the country's holding the Americans to use as a bargaining chip.

Sweden's Embassy in Pyongyang serves as protecting power for American citizens as the US and the North have has no diplomatic relations. But even when requested by the Swedish Embassy, the North "still routinely delays or denies consular access to US citizens," State Department spokeswoman Katina Adams said.

She said the North's recent detentions of US citizens highlight "the risk associated with travel to North Korea."

"North Korea routinely imposes unduly harsh sentences for actions that would not be considered crimes in the United States.

At least 16 US citizens have been detained in the DPRK in the past ten years," Adams said. "The Department of State strongly warns US citizens not to travel to North Korea."

On Sunday, the North's official Korean Central News Agency said the country has detained a Korean-American man, identified as Kim Hak-song, on suspicion of unspecified "hostile acts." His detention raised the number of US citizens held in the reclusive state to four.

KCNA said Kim had worked for Pyongyang University of Science and Technology.

Last week, the North announced that another American citizen, Kim Sang-dok, who taught at the same university, was arrested on April 22 for "committing criminal acts of hostility aimed to overturn" the country.

The other detainees are college student Otto Warmbier and Kim Dong-chul, a 63-year-old Korean-American.

Warmbier was sentenced to 15 years of hard labor for trying to steal a propaganda banner from a Pyongyang hotel, while Kim was sentenced to 10 years at hard labor on charges of espionage and subversion.

American visitors have often been detained in North Korea on charges of anti-state and other unspecified crimes. Widespread views are that the communist nation has used the detentions as bargaining chips in its negotiations with Washington. (Yonhap)