Lee Hae-chan, a veteran South Korean politician who played a quiet but consequential role in efforts to improve relations with North Korea, died Sunday at the age of 74.

Lee, who was serving as senior deputy chair of the Democratic Peaceful Unification Advisory Council, collapsed from a heart attack while visiting Vietnam’s Ho Chi Minh City, where he was attending meetings related to reunification, the council said.

South Korean President Lee Jae Myung wrote on social media that South Korea had “lost a great teacher in the history of its democracy.” 

Praising Lee’s lifelong commitment to peace on the Korean