U.S. President Donald Trump failed to strike progress with North Korea because he was too stingy when it came to easing sanctions, South Korea’s former unification minister said during a videoconference on Friday.

Kim Yeon-chul, the country’s unification minister until June 2020, stressed that the incoming Joe Biden administration must use sanctions more “flexibly” in order to build trust with Pyongyang. Kim resigned earlier this year after North Korea blew up the inter-Korean liaison office on June 16.

“During the Trump administration, there were shortcomings in negotiations with North Korea,” Kim said. “The tool of sanctions was used too