North Korea's top envoy, Kim Yong Chol, has arrived in Washington, DC, according to South Korean media, in a visit aimed at laying the groundwork for a second summit between US President Donald Trump and North Korean leader Kim Jong Un.

Neither the United States nor North Korea has announced any meetings, but Kim is expected to hold talks with US Secretary of State Mike Pompeo at a hotel in the US capital on Friday.

The meeting will likely be followed by a Kim visit to the White House, where he could meet with Trump, two officials told The Associated Press news agency, speaking on condition of anonymity because they were not authorised to discuss the matter publicly.

The North Korean official's arrival on Thursday - on a commercial flight from China's capital, Beijing - came on the same day as Trump unveiled a revamped US missile defence strategy that singled out Pyongyang as an ongoing and "extraordinary threat" - seven months after the US president declared after his first summit with Kim Jong Un that the North Korean threat had been eliminated.

Trump has spoken several times about having a second summit with Kim early this year and has exchanged multiple letters with the North Korean leader despite little tangible progress on a vague denuclearisation agreement reached at their meeting in Singapore last June.

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Since then, several private analysts have published reports detailing continuing North Korean development of nuclear and missile technology.

The little obvious progress was underlined by the new US Missile Defense Review that was unveiled on Thursday.

Introducing the report, Patrick Shanahan, the acting defence secretary, noted that North Korean missiles remained a "significant concern". Trump himself only mentioned North Korea in passing at the same event, saying negotiations he had conducted should have been done years ago.

Stalled talks

Yonhap and other South Korean media said Kim Yong Chol was greeted at Washington's Dulles airport by Stephen Biegun, the US special representative for North Korea.

His expected meeting with Pompeo on Friday will come months after planned talks between the two in November were called off at the last minute.

Contact between the two sides was resumed after a New Year's speech by North Korea's leader, in which he said he was willing to meet Trump "at any time", Cho Yoon-je, the South Korean ambassador to the US, told reporters last week.

The talks had stalled over North Korea's refusal to provide a detailed accounting of its nuclear and missile facilities that would be used by inspectors to verify any deal to dismantle them. North Korea has been demanding that the US lift harsh sanctions and provide it with security guarantees before it takes any steps beyond its initial suspension of nuclear and missile tests.

At a conference of US diplomats at the State Department on Wednesday, Vice President Mike Pence acknowledged the lack of progress.

He called the Trump-Kim dialogue "promising" but stressed that "we still await concrete steps by North Korea to dismantle the nuclear weapons that threaten our people and our allies in the region."

'Positive signs'

Al Jazeera's Florence Looi, reporting from South Korea's capital, Seoul, said the upcoming talks in Washington "were seen as restarting momentum that was lost on denuclearisation talks".

"This also comes as we've been getting other positive signals and a flurry diplomatic activity," added Looi.

Last week, South Korean President Moon Jae-in urged Pyongyang to take "bold, practical measures for denuclearisation" to ensure sanctions are lifted, but also stressed that "corresponding measures" were also needed from Washington, such as agreeing a "peace regime" and formally declaring an end to the 1950-1953 Korean War.

His comments came as Kim Jong Un visited China - North Korea's key ally - for talks with President Xi Jinping.

"It's thought that the meeting may have afforded the two leaders an opportunity to strategise ahead of a possible second Kim-Trump summit," Looi said.

A White House official, while not confirming plans for Friday's meeting, said "a lot of positive things" were happening related to North Korea's denuclearisation. The official said US-North Korea conversations were continuing, adding that the leaders of the two countries had established a "good relationship".

The official, who spoke to AP on condition of anonymity, said the two sides were "working to make progress" on the denuclearisation goal and that Trump "looks forward to meeting Chairman Kim again at their second summit at a place and time yet to be determined".