SEOUL, South Korea — An American who helped arrange for 30 female peace activists to cross the heavily armed border between North and South Korea in 2015 has been denied entry to South Korea, officials confirmed on Monday.

Christine Ahn, a South Korean-born American citizen, said she did not know she was persona non grata in the country until Asiana Airlines stopped her from boarding a flight at the San Francisco airport on Thursday. She had planned to transit through Incheon International Airport outside Seoul on her way to China, where she intended to spend a week before visiting South Korea.

After being told she was not allowed to transit though South Korea, she bought a new ticket to fly directly to Shanghai, she said.

The Justice Ministry of South Korea said on Monday that Ms. Ahn had been denied entry because there were sufficient grounds to fear that she might “hurt the national interests and public safety” of South Korea.