An International Atomic Energy Agency (IAEA) delegation plans to visit Pyongyang next week to discuss procedures for shutting down the North's Yongbyon reactor, which produces the raw material for bomb-making plutonium.
But a North Korean official in Vienna, where the UN nuclear watchdog is based, said his country had not yet given the formal go-ahead for the visit.
"The visit date of the delegation is not confirmed because the release of the frozen funds of DPRK (North Korea) at Banco Delta Asia in Macau has not been completed," embassy spokesman Hyon Yong-Man told reporters.
"Our side already informed the IAEA that we have no objections for the agency to prepare the visit as planned, but we are not ready to give our official confirmation for the scheduled visit of the agency due to the only reason of unfinished remittance."
The comments cast a cloud over apparent swift progress since Pyongyang said last Saturday the unfreezing of the funds "has reached its final phase," and invited an IAEA team to visit at an unspecified date.
The US envoy, arriving at the airport in steady rain, said his aim was to get the six-party talks process moving. The forum grouping the two Koreas, China, Russia, Japan and the United States reached a disarmament deal on February 13 but the cash row blocked any progress.
"We hope we can make up for some time we lost this spring," he said after being greeted by a smiling Ri Gun, director of the foreign ministry's America bureau.
China said its Foreign Minister Yang Jiechi would visit Pyongyang on July 2-4, with nuclear disarmament on the agenda. It described Hill's visit as positive.
"We hope it will be conducive to implementing the initial actions (of the February accord) and be of benefit to improving relations between North Korea and the US," said foreign ministry spokesman Qin Gang.
Choson Sinbo, a pro-North Korean newspaper published in Tokyo which often reflects official thinking, said the disarmament process may now speed up "if the US and the DPRK (North Korea) continue to build up mutual trust."
South Korea's foreign ministry said Hill would meet counterpart Kim Kye-Gwan, who extended the invitation, and vice foreign minister Kang Sok-Ju, who is close to reclusive leader Kim Jong-Il.
It said they would discuss a "road map" on implementing the February deal and the normalisation of bilateral relations.
The US State Department said Hill would leave Friday. He was to return to South Korea and go on to Tokyo on his way home.
In the February deal, the North agreed to disable its nuclear programmes in exchange for major aid and diplomatic benefits, including a possible normalisation of relations with Washington.
The pact followed a surge in tensions after the North carried out its first nuclear weapons test last October.
Four IAEA officials arrived in South Korea Thursday to discuss the mission scheduled for next week. A follow-up team will be sent to North Korea within weeks to verify the actual shutdown if it goes ahead.
It will be the inspectors' first visit since they were kicked out in late 2002.
Hill's trip is the first to North Korea by a top State Department official since October 2002, when his predecessor James Kelly confronted the North with alleged evidence of a secret nuclear programme using highly enriched uranium.
That accusation, and the North's denial, triggered off the latest nuclear crisis and the collapse of a 1994 bilateral denuclearisation accord.