US President Donald Trump on Monday urged China to maintain tight control of its border with North Korea until he signs a denuclearization deal with Kim Jong Un.

Trump is scheduled to hold a landmark summit with the North Korean leader June 12 in Singapore, but Pyongyang recently has threatened to pull out over US demands for "unilateral nuclear abandonment."

"China must continue to be strong & tight on the Border of North Korea until a deal is made," Trump tweeted, suggesting China may have eased up on enforcing economic sanctions against Pyongyang.

"The word is that recently the Border has become much more porous and more has been filtering in.", he wrote, adding he wants North Korea to be 'VERY successful,' but only once a deal has been reached on denuclearization," he said.

China is North Korea's biggest trade partner, and Trump has called on it repeatedly to press Pyongyang to rein in its nuclear and missile programs.

Besides protesting US disarmament demands, North Korea also angrily condemned joint US-South Korean military exercises, and last week pulled out of planned inter-Korean talks.

Trump's National Security Advisor John Bolton also has drawn Pyongyang's ire by referring to Libya as a denuclearization model. After giving up Libya's nuclear program in 2003, leader Moamer Kadhafi was killed in a NATO-backed uprising in 2011.

North Korea's sudden shift in attitude followed a weeks-long charm offensive that saw Kim Jong Un hold a historic summit in the Demilitarized Zone with South Korean President Moon Jae-in, and meet twice with China's President Xi Jinping.

As prospects for the Singapore summit dimmed, Trump offered assurances that if the summit is successful, Kim "will get protections that will be very strong."

"He'd be in his country and running his country. His country would be very rich," he told reporters last week.