Ms. Ahn said she suspected that the government of the former president Park Geun-hye, a conservative who was impeached over a corruption scandal and removed from office in March, had put her on a blacklist for helping organize the Women Cross DMZ campaign in May 2015.
In that campaign, a group of 30 female peace activists, including Gloria Steinem and two Nobel Peace Prize laureates, visited North Korea and then crossed the Demilitarized Zone, or DMZ, into South Korea, calling for a treaty to formally end the Korean War.
“I’m hoping the Moon administration can quickly correct this,” Ms. Ahn wrote in an email from China, referring to President Moon Jae-in, who replaced Ms. Park in May. “It would not look favorably if they’re allowing the dark ages to remain by banning pro-democracy, human rights and peace activists from entering their country.”
While in office, Ms. Park, who took a hard line on North Korea and its nuclear weapons program, had prevented South Korean nongovernmental organizations from making contact with the North. In 2015, her government deported a South Korean-born American citizen who had spoken warmly about her trips to North Korea in public appearances and articles in South Korea.
Some critics in South Korea and the United States opposed the Women Cross DMZ campaign, saying the activists’ trip to North Korea would be used as propaganda by the North’s repressive government. North Korean state media later quoted some of the activists as praising the country’s leaders, but organizers denied that the remarks had been made.
Ms. Ahn, 44, has been an international coordinator for the campaign. On Sunday, two of its members, Ms. Steinem and the Nobel Peace Prize laureate Mairead Maguire, sent letters to Ahn Ho-young, the South Korean ambassador to Washington, urging Seoul to lift the ban on Ms. Ahn.
Ms. Ahn said she was now in China with other female peace activists from the United States and Canada. She had planned to fly to Seoul next Monday, together with another antiwar activist, Mary Ann Wright, a retired United States Army officer and former diplomat.
Ms. Ahn said that Ms. Wright still plans to go to Seoul, where she and other female activists from the United States, Switzerland, Japan and Australia plan to campaign for a peace treaty to officially end the 1950-53 Korean War. Ms. Ahn said there were no indications that Ms. Wright or any of the other activists would be barred from South Korea.
In a speech on July 6, President Moon said his government would push for a peace treaty with North Korea, as part of a comprehensive solution to the crisis over the North’s nuclear weapons program. The Korean War was halted with a cease-fire that left the peninsula still technically at war.