The United States wants to ensure that progress in denuclearising North Korea is not outpaced by efforts to improve inter-Korean ties, US Secretary of State Mike Pompeo has said.

Pompeo's remark on Tuesday underscores US concerns that a recent flurry of inter-Korean activity could undermine sanctions placed on the North for its nuclear weapons and ballistic missile programmes.

"We have made clear to the Republic of Korea that we do want to make sure that peace on the peninsula and the denuclearisation of North Korea aren't lagging behind the increase in the amount of inter-relationship between the two Koreas," Pompeo said in a news conference, South Korean News Agency (Yonhap) reported.

"We view them as tandem, as moving forward together. We view them as important parallel processes."

South Korean and American officials were meeting in Washington the same day to launch a working group that will help coordination on the matter.

Pompeo said there is "complete agreement" between the sides on how the two efforts should proceed.

And the working group is designed to ensure that "we can be sure that we don't talk past each other, that we don't take an action, or the South Koreans don't take an action that the other is unaware of or hasn't had a chance to comment on, or provide their thoughts," according to the top US diplomat.

The working group is led on the South Korean side by Lee Do-hoon, special representative for Korean Peninsula peace and security affairs, and on the US side by Special Representative for North Korea Stephen Biegun.

Of interest is whether the two sides will resolve lingering issues over the proposed reconnection of an inter-Korean railway.

A joint field survey of the railroad linking Seoul to the North Korean city of Sinuiju was scheduled to begin late last month, but the US has apparently voiced concerns that such projects could dampen United Nations sanctions on the regime.

South Korea has been eager to expand cross-border exchanges in a bid to spur the North's denuclearisation, a commitment made by North Korean leader Kim Jong-un at his historic summits this year with South Korean President Moon Jae-in and US President Donald Trump.

While formal negotiations between Washington and Pyongyang have stalled, Trump has said he expects to have a second summit with Kim early next year.

South Korea has also said it expects Kim to visit Seoul before the end of the year in line with an agreement reached during the last inter-Korean summit in September.