SEOUL (Reuters) - Pyongyang University of Science and Technology (PUST), North Korea s only western-funded university, will start the fall semester without its dozens of American staff after failing to secure exemptions to a U.S. travel ban that starts on Friday.  PUST - home to the largest concentration of foreigners in the reclusive state - plans to revise courses and teaching schedules but its largely English-based curriculum will be heavily impacted, two sources familiar with PUST s operations said. PUST was founded in 2010 by a Korean American evangelical Christian with the goal of helping North Korea's future elite learn the skills to modernize the isolated country and engage with the outside world. (Graphic: Americans detained in North Korea - tmsnrt.rs/2pmE3ks) In mid-July, however, the U.S. State Department announced a ban on Americans traveling to North Korea following the death earlier this year of an American student who had been detained by the state while on a tour. It advised U.S. citizens living there to leave. Since then, tensions on the Korean peninsula have escalated significantly. Nuclear-armed North Korea has undertaken a number of provocative missile tests, including two intercontinental ballistic missile launches and one medium-range missile test this week that flew over Japan. Of the roughly 130 foreigners at PUST including faculty members, staffers and family members, about 60 were U.S. citizens, one of the sources said, asking not to be named due to the sensitivity of the situation. None had received special permission to stay and all have now left Pyongyang.  The teaching activities and the unique  international English-based  character of the school are severely impacted by the U.S. travel ban and the decision of some other personnel not to return,  the source said.  The school, which is open about its Christian affiliation, has already been rocked by the detention of two of its staff members by the North Korean regime this year. The two were accused of acting against the interests of the state. U.S. officials announced the ban six weeks ago due to the risk Americans will be held for  long-term detention  in the country. The ban was imposed after U.S. student Otto Warmbier was sentenced to 15 years hard labor for allegedly trying to steal a propaganda poster while on a tour last year.  Warmbier fell into a coma in custody and died soon after he was released to U.S. officials. The circumstances surrounding his death are not clear. The last major tour involving U.S. tourists flew out of North Korea on Thursday. The ban, similar to previous U.S. restrictions on travel to Iraq and Libya, makes North Korea the only country in the world Americans are currently banned by the State Department from visiting. Journalists and humanitarian workers can apply for special permission. But the U.S. government has yet to issue guidelines on how to obtain waivers, leaving PUST and educators uncertain how or when it might be possible to apply for exemptions, according to the people familiar with PUST s operations. The State Department said it was unable to comment on specific requests for exemptions.  Over the past several weeks, PUST leaders have tried to lobby for exemptions so that their work can continue, the sources said. The school is involved in ongoing discussions with U.S. officials, they added. The university was seen as a rare experiment in academic diplomacy with a country increasingly entirely isolated from the rest of the world due to tightening sanctions over its defiant pursuit of nuclear weapons programs. U.S. President Donald Trump has vowed to stop Pyongyang achieving its goal of being able to fire a nuclear warhead to the United States and has warned the U.S. military is  locked and loaded  in case of North Korean provocation. Since PUST took in its first 50 students in 2010, the school has grown to about 500 undergraduate and 60 graduate students studying in mostly three departments - electronic and computer engineering, international finance and management and agriculture and life sciences.   A lot of effort has been spent over the past seven years that PUST has been operating. The prospect of this progress being undermined - even totally thwarted - as a consequence of the U.S. government action, is deeply worrying to everyone involved,  one of the sources said. 