The White House press secretary, Sarah Huckabee Sanders, said on Friday that certain conditions would need to be met before President Donald Trump would sit down with North Korean leader Kim Jong Un.

Sanders said Trump would take the meeting only "on the basis that we have concrete and verifiable steps" toward the denuclearization of North Korea.

Trump agreed to meet with Kim in the next two months, South Korea's national security adviser said on Thursday.

The White House said on Friday that certain conditions would need to be met before President Donald Trump would meet with North Korean leader Kim Jong Un.

"The president will not have the meeting without seeing concrete steps and concrete actions take place by North Korea," said Sarah Huckabee Sanders, the White House press secretary. "So the president would actually be getting something."

She said later in the briefing that Trump would take the meeting only "on the basis that we have concrete and verifiable steps" toward the denuclearization of North Korea.

South Korea's national security adviser, Chung Eui-yong, said at the White House on Thursday that Trump had agreed to "meet Kim Jong Un by May to achieve permanent denuclearization."

Sanders said that neither a date nor a location had been determined for a meeting, to which Trump seems to have already committed.

The White House's reaction had North Korea observers questioning whether the meeting would even take place.

"If they're this uncomfortable talking about a prospective summit, it probably means it won't happen by May and may not even happen at all," Ankit Panda, a senior editor at The Diplomat, wrote on Twitter.

Other US officials have pushed back on Sanders' comments.

"The invitation has been extended and accepted, and that stands," an unidentified White House official told a Wall Street Journal reporter.