President Trump arrives to speak at a prison reform summit in the East Room at the White House on Friday. (Jabin Botsford/The Washington Post)

Amid uncertainty over a planned U.S.-North Korea summit, President Trump pushed Monday for China to continue “strong & tight” trade sanctions against Kim Jong Un’s regime until a possible accord over the North’s nuclear program.

“China must continue to be strong & tight on the Border of North Korea until a deal is made,” Trump wrote on Twitter. “The word is that recently the Border has become much more porous and more has been filtering in.”

Trump added: “I want this to happen, and North Korea to be VERY successful, but only after signing!”

Trump and Kim are scheduled to meet for their highly touted summit June 12 in Singapore.

But the plans remain clouded after Pyongyang threatened last week to cancel the summit, and the White House continued to assess the political risks for Trump if he falls short of a sweeping deal to denuclearize the Korean Peninsula.

China is North Korea’s main trade partner, accounting for approximately 90 percent of trade with the rogue state, and its cooperation in enforcing economic sanctions is considered key to pressuring the country into an agreement. Trump has long sought to convince China to work with the United States on a “maximum pressure” strategy of persuading North Korea to abandon its nuclear program.

Inside the White House, there are mounting concerns that North Korea may back away from striking a deal on denuclearization and growing suspicions about the role the Chinese may be playing in complicating preparations for the Singapore summit.

An advance team from the United States has been in Singapore working out logistics and trying to nail down an itinerary, even as the national security adviser, John Bolton, and other top Trump aides privately have voiced pessimism that the summit will be successful.

On Saturday evening, Trump spoke to South Korean President Moon Jae-in for guidance in interpreting Kim’s harder-line position in the weeks since the two Korean leaders struck a positive tone during their landmark meeting.

Trump attributes the shift in part to the influence of the Chinese, saying last week that he thinks the North Korean posture changed after Kim’s recent visit to Beijing to meet with Chinese President Xi Jinping. It was Kim and Xi’s second meeting in recent months, and came as a surprise to the Americans.

“There has been a big difference since they had the second meeting with President Xi,” Trump said last Thursday, calling it “a little bit of a surprise meeting.”

“With that being said, my attitude is, whatever happens, happens,” Trump added. “Either way, we’re going to be in great shape.”

In separate tweets Monday, Trump defended his approach to trade with China amid criticism for apparently bending to Beijing in key areas of dispute.

On Sunday, Treasury Secretary Steven Mnuchin said that the administration put its trade war with China “on hold” after two days of talks in Washington. He said the meetings produced an agreement on increased Chinese purchases of American products and measures to make it easier for U.S. companies to operate in China.

Trump tweeted that the agreement would be “one of the best things to happen to our farmers in many years!”

“Under our potential deal with China, they will purchase from our Great American Farmers practically as much as our Farmers can produce,” Trump asserted.

In an earlier sign of softening, Trump directed administration officials to consider easing harsh penalties on a prominent Chinese telecom company that had violated U.S. sanctions on Iran and North Korea.

David J. Lynch contributed to this report.