WASHINGTON (Reuters) – Republican and Democratic U.S. congressmen introduced a bill on Thursday that would ban Americans from traveling to North Korea as tourists and require them to obtain special permission for other types of visits.

Democrat Adam Schiff and Republican Joe Wilson said their proposed North Korea Travel Control Act followed the detention of at least 17 Americans in North Korea in the past decade.

North Korea has a record of using detained Americans to extract high-profile visits from the United States, with which it has no formal diplomatic relations.

“With increased tensions in North Korea, the danger that Americans will be detained for political reasons is greater than ever,” the congressmen said in a statement.

Given North Korea’s “demonstrated willingness to use American visitors as bargaining chips to extract high level meetings or concessions, it is appropriate for the United States to take steps to control travel to a nation that poses a real and present danger to American interests,” they said.

Four Americans are being held in North Korea as diplomatic tensions with Washington have heightened. Two of them, detained in the past month, are affiliated with a private university in the North Korean capital.

A congressional source said the bill would ban tourist travel by Americans outright, while any other visits would require a special license from the Treasury Department, which is enforcing a wide range of sanctions on North Korea over its nuclear and missile programs.

North Korea this month asserted its sovereign right to “ruthlessly punish” U.S. citizens it has detained for crimes against the government. It said calling such arrests bargaining ploys was “pure ignorance.”

North Korea said on May 7 it had detained Kim Hake Song, who worked for the Pyongyang University of Science and Technology, on suspicion of “hostile acts.”

Another American, Kim Sang Dok, who was associated with the same school, was detained in late April on the same charge.

The other two Americans are Otto Warmbier, a 22-year-old student detained in January 2016 and sentenced to 15 years hard labor for attempting to steal a propaganda banner, and Kim Dong Chul, a 62-year-old Korean-American missionary.

Kim was sentenced to 10 years hard labor for subversion last year.

(Reporting by David Brunnstrom; Editing by Richard Chang)