US envoy Christopher Hill has made his first trip to North Korea amid speculation that talks on Pyongyang's nuclear programme could soon restart.
Christopher Hill has never visited Pyongyang before
Optimism at the visit has been dampened by reports that a planned trip by UN nuclear inspectors may not go ahead.
A North Korean spokesman said the visit would only be confirmed when funds frozen in a Macau bank were released.
The US said earlier this week that the money had already been transferred, but the North appears to think otherwise.
Russian officials gave assurances on Thursday that $25m (£12.5m) was being transferred to Pyongyang's accounts.
However, Pyongyang could not confirm the international delegation's visit until it had verified the transfer.
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The inspectors are due to arrive in Pyongyang next week - their first visit since they were forced out of the country in 2002.
"Our side has informed the IAEA [UN nuclear watchdog] that we have no objection to them preparing the visit as a plan, but we are not [yet] ready to give our official confirmation for the visit," a North Korean diplomat said in Vienna.
Mr Hill left for Pyongyang from an air force base near Seoul early on Thursday.
The two-day visit was expected to focus on an agreement to shut down North Korea's Yongbyon reactor in exchange for economic aid.
Progress on the February deal has been slow, largely due to the row over North Korean funds frozen in a Macau back.
North Korea has pledged to shut down its main reactor at Yongbyon
Mr Hill announced on Tuesday that the money had finally been delivered to a North Korean bank account in Russia, sparking hopes that talks on the North's nuclear programme could soon restart.
The US state department has denied that the visit signals a major shift in US policy, but the BBC's Jonathan Beale in Washington says Mr Hill's presence in the country will be seen as a positive sign.
Washington has long rejected bilateral negotiations with North Korea, favouring six-party talks involving the whole region.
Mr Hill is the most senior US official to visit the country in five years.
The new urgency is a result of North Korea's test of a nuclear device last October.