'Maximum pressure' campaign against North Korea worked: White House

The Trump administration's "maximum pressure" campaign against North Korea has worked, the White House has said, ahead of President Donald Trump's historic summit with North Korean leader Kim Jong-un.

The White House statement comes a day after North Korea released three Americans who were detained in the country for months.

"The President has talked about, certainly, that the maximum pressure campaign has worked. We have seen North Korea make progress since that campaign started," White House Press Secretary Sarah sanders told reporters.

Sanders said Trump had also been very clear that some of the success was due to America's key allies and partners like South Korea's President Moon Jae-In, Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping, who played a "big role" putting pressure on North Korea.

"They have all played a big role in the maximum pressure campaign. But a lot of that is due to the fact that the President has spent time developing relationships with those individuals, and they've collectively been able to put pressure on North Korea," Sanders said on Wednesday.

Trump told reporters on Wednesday that the time and venue for his summit meeting Kim had been decided and would be announced in three days, ruling out the Demilitarised Zone that divides the Korean peninsula as a venue for the meeting.

Trump had spoken with Moon Jae-in to discuss Pompeo's visit to North Korea.

"The two leaders welcomed North Korea's decision to release three American detainees, and expressed hope for joyful family reunions," said a readout of the call.

Trump would host Moon at the White House on May 22.

Sanders said the President viewed the release of three prisoners by North Korea as a positive gesture and a step in the right direction from the North Korean leader.

"However, total denuclearisation will remain the top priority," she said.

Congresswoman Tulsi Gabbard called for building on the momentum.

"The nuclear threat posed to us by North Korea is real, and the need to denuclearise North Korea is urgent. Historic progress has already been made.

"Those calling for delays or setting preconditions before direct talks between President Trump and Chairman Kim are grossly out of touch with and naive to the seriousness of the threat that the people of Hawaii live with every day," she said.

"We cannot keep kicking the can down the road. We must build on the momentum of the historic summit between North and South Korean leaders, and do all we can toward a successful US-North Korean summit to denuclearise North Korea," Gabbard said.