The return of IAEA inspectors, expelled in 2002, is part of the February 13 deal to give the impoverished state energy aid in return for ending its nuclear weapons programme.
"In an offhanded way, a DPRK [North Korean] official mentioned that perhaps 30 additional days would be needed because of the current delay" prompted by the issue of frozen funds, Richardson said.
"We think now it is really an important time to get on with the ever-urgent task of denuclearisation, and particularly get on with the implementation of the February agreement"
Christopher Hill, US chief negotiator with North Korea
Sean McCormack, US state department spokesman, said the US and other members of the six-party talks on North Korea would check Pyongyang's progress on Saturday.
"We would expect the North Koreans to act in such a way that they meet their obligations under the Feb 15 agreement," McCormack told reporters.
The US said on Tuesday that Macau authorities had unblocked about $25m of frozen North Korean funds at Banco Delta Asia (BDA) in the southern Chinese city.
Richardson said he expected the bank to tell North Korea it could collect its money on Wednesday or Thursday, meaning Pyongyang would send an invitation to the IAEA on Friday.
BDA officials in Macau declined to comment on whether the funds had been transferred or whether North Korea had been in contact.
Christopher Hill, US chief negotiator with North Korea, told reporters in Seoul he considered the bank issue resolved.
"We think now it is really an important time to get on with the ever-urgent task of denuclearisation, and particularly get on with the implementation of the February agreement," he said.