A former NATO secretary general said Monday that he has been blocked from entering the U.S. over a visit to Iran in 2013.

Javier Solana told The New York Times that his renewal application on the Electronic System for Travel Authorization (ESTA) was rejected for the first time. The system allows individuals from some countries to travel to the U.S. without having to apply for a visa.

Solana, a Spanish citizen, told the Times that he believed the rejection “to be more bureaucratic than political.”

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The Obama administration in 2016 tightened restrictions for ESTA, blocking citizens of 38 countries who traveled to Iran, Iraq, Sudan or Syria after March 1, 2011, from using the program. Citizens of those countries are also not permitted to use it.

Those individuals must instead go through the process to obtain a U.S. visa. Waivers can be granted on an individual basis for people who traveled to Iran as humanitarian workers, journalists or government representatives.

Solana said he traveled to Iran in 2013 to attend the inauguration of President Hassan Rouhani. He added that he will apply for a U.S. visa.