US Secretary of State Mike Pompeo says he is confident talks with North Korean officials are moving in the right direction toward a summit and that a North Korean envoy will travel to Washington to deliver a personal letter from leader Kim Jong-un to President Donald Trump.
"Our two countries face a pivotal moment in our relationship in which it could be nothing short of tragic to let this opportunity go to waste," Mr Pompeo said after meetings with North Korean vice chairman Kim Yong-chol in New York.
"We have made real progress toward that in the last 72 hours."
Kim Yong-chol is a close aide of Kim Jong-un and is vice-chairman of the ruling Workers' Party's Central Committee.
Mr Trump played down the chances of a quick breakthrough in nuclear talks with North Korea.
In a brief interview with Reuters aboard Air Force One on the way to Texas he said he was still hoping to hold an unprecedented meeting with Kim Jong-un on June 12 in Singapore.
"I'd like to see it done in one meeting," he said.
"But often times that's not the way deals work. There's a very good chance that it won't be done in one meeting or two meetings or three meetings.
"But it'll get done at some point."
Mr Pompeo said a North Korean delegation, headed by high-ranking official Kim Yong-chol, would make a rare visit to the White House on Friday (local time) and give Mr Trump a letter from leader Kim.
The letter appeared to be in response to a comment from Mr Trump when he cancelled the summit, accusing Pyongyang of hostility, but urged the North Korean leader to "call me or write" if he had a change of heart.
North Korea, whose nuclear ambitions have been a source of tension for decades, has made advances in missile technology in recent years but Mr Trump has sworn not to allow it to develop nuclear missiles that could hit the United States.
He wants North Korea to "denuclearise," meaning to get rid of its nuclear arms, in return for relief from economic sanctions but the leadership in Pyongyang is believed to regard nuclear weapons as crucial to its survival and has rejected unilaterally disarming.
The North Korean visit to the White House would be the first there by a high-level Pyongyang official since 2000 when senior figure Jo Myong-rok met President Bill Clinton.
Mr Pompeo's talks with Kim Yong-chol — the most critical of three tracks of negotiations taking place between the two governments in the US, in the heavily fortified Korean Demilitarised Zone, and in Singapore — are aimed at determining whether a meeting between Mr Trump and Kim Jong-un, originally scheduled for June 12 but later cancelled by Mr Trump, can be restored.Kim jong-un complains of 'US hegemonism'
But Kim Jong-un, in a meeting with Russia's foreign minister on Thursday, complained about "US hegemonism," a comment that may complicate the discussions in New York.
He told Mr Lavrov he hoped to boost cooperation with Russia, which has remained largely on the sidelines in recent months as Mr Kim made a major diplomatic outreach to the United States as well as to South Korea and China.
"As we move to adjust to the political situation in the face of US hegemonism, I am willing to exchange detailed and in-depth opinions with your leadership and hope to do so moving forward," he told Mr Lavrov.
Mr Lavrov's visit to Pyongyang suggests that Russia wants to become involved and make sure North Korea informs it of its intentions and is mindful of Moscow's concerns.
In their talks, Mr Lavrov relayed Mr Putin's "warmest regards and best wishes" for Mr Kim's "big endeavours" on the Korean Peninsula.
He also expressed Moscow's support for an agreement Mr Kim reached with Mr Moon at a summit last month that focused on measures to ease hostilities and increase exchanges between the two Koreas.