The United States will soon ban its citizens from traveling to North Korea, two China-based tour operators said Friday. The official announcement from Washington is expected July 27 and the ban will be implemented 30 days later, according to the operators.

“We have just been informed that the U.S. government will no longer be allowing U.S. citizens to travel to the DPRK [Democratic People’s Republic of Korea]. It is expected that the ban will come into force within 30 days of July 27. After the 30 day grace period any U.S. national that travels to North Korea will have their passport invalidated by their government,” Young Pioneer Tours said in a statement.

Read: Otto Warmbier Dies And Family Blames North Korea For Death

According to Reuters, Koryo Tours also announced the U.S. will implement a travel ban for North Korea. It remains unclear how long the ban will last.

Young Pioneer Tours was the same tour operator through which U.S. student Otto Warmbier went to the isolated nation. He was sentenced to 15 years imprisonment with hard labor after being convicted by North Korean officials for stealing a propaganda poster from his hotel last January. Warmbier was released June 13 in a comatose state, which Pyongyang blamed on botulism and a sleeping pill. The 22-year-old died June 19 and his death triggered widespread criticism from U.S. officials against the Kim Jong Un-led regime. Following the incident, Young Pioneer Tours said it would not accept U.S. nationals for North Korea tours.

The U.S. has always discouraged its citizens from traveling to the country, citing the risk of arrest and long-term detention.

The latest travel advisory issued by the Department of State dated May 9, said: “North Korean authorities have detained those who traveled independently and those who were part of organized tours. Being a member of a group tour or using a tour guide will not prevent North Korean authorities from detaining or arresting you.”

“Since the United States does not maintain diplomatic or consular relations with North Korea, the U.S. government has no means to provide normal consular services to U.S. citizens in North Korea,” the advisory continued. “Possession of any media, either physical or electronic, that is critical of the DPRK government or its leaders is considered a criminal act punishable by long-term detention in hard labor camps and heavy fines,” it warned.

Read: Did North Korean Defector Willingly Return To Kim Jong Un's Country?

North Korea is known to scrutinize the activity of tourists. Following Warmbier’s release, the current count of U.S. nationals detained in North Korea stands at three — businessman Kim Dong Chul, and professors Kim Sang Duk and Kim Hak Song.

Kim Dong Chul was arrested in October 2015 and was sentenced to 10 years hard labor in April 2016 for espionage. A professor at Yanbian University of Science and Technology in China, Kim Sang Duk was arrested this April on charges of attempting to overthrow the North Korean government, CNN reported in May. Kim Hak Song, a Korean-American professor at Pyongyang University of Science and Technology, was arrested in May and was reportedly charged with suspicion of “hostile acts” towards the regime.