FILE - In this Sept. 19, 2017, file photo, President Donald Trump addresses the 72nd session of the United Nations General Assembly, at U.N. headquarters. President Trump accepting a reported offer to meet with North Korean leader Kim Jong Un is a stunning turn of events after a year that saw them engage in a heated verbal warfare that included crude insults and mutual threats of nuclear attacks. It remains to be seen whether a summit will take place or lead to a meaningful breakthrough in the nations’ relationship. (AP Photo/Richard Drew, File)

WASHINGTON - The United States says it will not impose more conditions on North Korea before a summit of the two nations' leaders, beyond the North's promise not to resume nuclear and missile tests or publicly criticize U.S.-South Korean military exercises.

But the uncertainty still lingers over plans for the first-ever meeting of a serving U.S. president and a North Korean leader. Four days after the surprise announcement that President Donald Trump has agreed to meet the North's Kim Jong Un by May, Washington has yet to hear directly from Pyongyang on the invitation extended by Kim via South Korean intermediaries.

"It's very early stages," Secretary of State Rex Tillerson told reporters Monday in Abuja, Nigeria, on a swing through Africa, when asked about plans for the meeting, expected in May. "We've not heard anything directly back from North Korea, although we expect to hear something directly from them."

He said that no venue for the meeting had been agreed upon. "I think it's going to be very important that those conversations are held quietly" between Washington and Pyongyang," Tillerson said.

A summit between two nations that have remained in a state of war since the Korean War would be a remarkable turnabout after a year of heightened tensions. Kim has pushed his isolated nation closer to having a nuclear-tipped missile that can strike the U.S. mainland. Trump has vowed to prevent that, by force if necessary.

On Friday, White House press secretary Sarah Huckabee Sanders said the meeting wouldn't take place "until we see concrete actions that match the words and the rhetoric of North Korea." But Deputy White House spokesman Raj Shah said on Sunday that no additional conditions are being stipulated.

He stressed that North Korea must stick to the commitments that were relayed by South Korean officials who met with Kim last week and then came to Washington and briefed Trump. Shah told ABC's "This Week" that the North Koreans "cannot engage in missile testing, they cannot engage in nuclear testing and they can't publicly object to the U.S.-South Korea planned military exercises."

The U.S. and South Korea hold military manoeuvrs every year. They were postponed during the recent Winter Olympics in South Korea which provided an impetus for the current diplomatic push on the divided Korean Peninsula. The drills are expected to be held in April, but no official announcement has been made about when they will take place. In an interview en route to the Middle East, Defence Secretary Jim Mattis declined to discuss the timing and scale of the exercises.

Mattis wouldn't talk at all about the diplomatic push over North Korea's nuclear program. "When you get in a position like this, the potential for misunderstanding remains very high," he said.

Mattis was among the advisers in the White House on Thursday when Trump decided to accept Kim's offer to meet.

Trump said Saturday that he believes North Korea will abide by its pledge to suspend missile tests while he prepares for the summit. He noted in a tweet that North Korea has refrained from such tests since November and said Kim "has promised not to do so through our meetings."

"I believe they will honour that commitment," the president said.

Later, at a political rally in Pennsylvania, when Trump mentioned Kim's name, the crowd booed. But Trump responded: "No, it's very positive ... no, after the meeting you may do that, but now we have to be very nice because let's see what happens, let's see what happens."

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Sherman reported from Washington and Josh Lederman from Abuja, Nigeria. Associated Press writer Roberts Burns contributed from Muscat, Oman.

News from © The Associated Press, 2018