South Korea's chief nuclear negotiator is travelling to the US to try to resolve a major stumbling block to North Korea's nuclear disarmament.
Unblocking the BDA accounts is key to progress, the North has said
Chun Yung-woo said he did not want to see a dispute over North Korean bank accounts scupper progress towards ending the North's nuclear programme.
Washington has lifted a freeze on the North's accounts, but Pyongyang appears to be unable to access the money.
On Sunday South Korea agreed to resume food aid shipments to the North.
Following five days of talks in Pyongyang, Seoul said it would begin delivering 400,000 tonnes of rice to its impoverished neighbour.
While no reference was made to the North's nuclear programme in the final communique at the talks, Seoul has insisted the aid is linked to progress on disarmament.
Unification Minister Lee Jae-joung reiterated the South's position on Monday, saying the aid was dependent on whether the North fulfilled its pledge to begin the process of dismantling its nuclear programme.
"The rice issue is not just a humanitarian issue, but a very symbolic and essential task for peace," he told MBC radio.
The North missed a mid-April deadline - agreed on 13 February between the two Koreas, Japan, China, Russia and the US - to "shut down and seal" its Yongbyon reactor in return for aid.
Pyongyang made clear it would only close the reactor if $25m (£13m) of its money frozen in the Macau-based bank Banco Delta Asia (BDA) was returned.
The US has said the accounts are now unfrozen, and insists it does not know why the North has left the funds untouched.
South Korea's Chun Yung-woo said his talks with US counterpart Christopher Hill in Washington would focus on "technical issues" over the banking dispute.
"We cannot continue putting off the more important denuclearisation issue because of this BDA issue," he said before leaving Seoul.
He said the North's demands had "generally been identified", but more time was needed to fully resolve the issue, Yonhap news agency reports.
"Let us wait and see for a little longer because the parties are working hard for the resolution," he said.
N KOREA NUCLEAR DEAL
N Korea to 'shut down and seal' Yongbyon reactor, then disable all nuclear facilities
In return, will be given 1m tonnes of heavy fuel oil
N Korea to invite IAEA back to monitor deal
Under earlier 2005 deal, N Korea agreed to end nuclear programme and return to non-proliferation treaty
N Korea's demand for light water reactor to be discussed at "appropriate time"
The nuclear issue, as well as rice aid, was central to intense negotiations between the two Koreas, which went into an unscheduled fifth day on Sunday.
Seoul, a major food donor to its northern neighbour, suspended aid after Pyongyang's missile tests in July 2006, which was followed by a nuclear test in October.
The BBC's Charles Scanlon in Seoul says Pyongyang badly needs the aid, because stocks from last year's harvest are running out.
The first rice shipments are due to begin arriving in the North in May.