President Donald Trump made history when meeting with North Korean leader Kim Jong-un, but there still is a long road ahead before his goals are achieved, experts testified Wednesday.

The House Asia and the Pacific Subcommittee held a hearing to review the outcomes of the Singapore summit on June 12. The panel of experts expressed concern over the lack of substance but were hopeful it was the first step towards something more substantial, and detailed what it might take to get there.

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“We need to maintain pressure until significant progress is achieved,” Bruce Klingner, the senior research fellow for Northeast Asia at The Heritage Foundation, said. “U.S. negotiators should make clear the differences between negotiable presanctions such as constraints on resources, import and export in the U.N. resolution, and non-negotiable U.S. targeted financial measures, which are law enforcement mechanisms.”

Related: Here Are the Experts’ Must-Do’s for Denuclearizing Korea

North Korea poses several problems with its nuclear program at the forefront of the administration. There are human rights and economic issues, many of which the administration has been urged to address. North Korea also has a history of ignoring past agreements, like those formed during the six-party talks.

“We’ll know it’s different this time, or potentially different, if Secretary [Mike] Pompeo can get the North Koreans to produce a list of North Koreans’ weapons and missiles and a verification plan,” Michael Green, the senior vice president for Asia at the Center for Strategic and International Studies, said. “We were supposed to get this during the six-party talks.”

Green adds that the administration will also need to maintain support from our allies, both when it comes to North Korea and China’s efforts to advance its power in the region. Abraham Denmark, the director for Asian policy at The Woodrow Wilson International Center for Scholars, warned that time is also a major factor with North Korea’s continuing to develop its nuclear capabilities, though they agreed to freeze testing.

Related: Trump Withdraws from North Korea Summit

“One way for the United States to address time pressures in this negotiation would be to achieve a complete freeze on North Korea’s missile programs,” Denmark said. “The U.S. should insist on strict inspection and verification mechanisms before coming to any concessions focused on denuclearization.”

Connor Wolf covers Congress and national politics and can be reached at [email protected]. Follow him on Twitter.