Former U.S. President Barack Obama will not travel to Nagasaki in August to attend a peace conference there, officials of the organizer said Thursday.

Mayors for Peace, a Hiroshima-based nongovernmental organization, had invited the former president to attend the forum but received an email from the Obama Foundation in mid-April saying he would not be able to visit Nagasaki, citing his tight schedule.

The group, which promotes nuclear disarmament and world peace, had wanted Obama to visit the city, which was devastated by a U.S. atomic bomb on Aug. 9, 1945, and give a keynote speech at its general meeting.

In May last year, Obama became the first sitting U.S. president to visit Hiroshima, site of the first U.S. atomic bombing three days before Nagasaki.

Over 7,000 cities in 162 countries and regions are expected to join the peace conference to be held in Nagasaki on Aug. 7-10. It convenes a general conference once every four years.