Diplomatic hopes were raised after North Korean leader Kim Jong-il told a South Korean envoy last month that Pyongyang could rejoin the six-party talks in July if the United States met certain conditions, such as showing respect.
"A lot of effort has been made to create favourable conditions for the North's participation in the six-party talks. We believe North Korea knows that, and we expect the resumption of the talks in July," Foreign Minister Ban Ki-moon said on Wednesday.
He said US Secretary of State Condoleezza Rice's trip to East Asia, which is scheduled to start on Friday, would add momentum to resuming the talks.
North Korea said in February that it possessed nuclear weapons and was boycotting the talks conducted by the two Koreas, China, Japan, Russia and the United States.
A series of contacts by Seoul and Washington with Pyongyang's envoys in past weeks have fuelled expectation that it was a matter of time before the North announced its return to the talks that have stalled for a year.
However, Japan's foreign minister said on Monday that he did not share that optimism.
"In some quarters there is a very optimistic view and they probably have their basis for that, but the Japanese government is neither extremely optimistic nor extremely pessimistic," Foreign Minister Nobutaka Machimura said in an interview.
Seoul hopes Condoleezza Rice's
trip to Asia will build momentum
Asked how long Japan and other countries were prepared to wait, he said: "Soon, we will reach the limits of our patience."
A prominent South Korean analyst on North Korea's security strategy said it was too optimistic to anticipate a quick resumption of the talks and the North agreeing to a negotiated settlement on ending its nuclear programme.
"At every step of the way in the six-party process, the outlook just hasn't been all that bright," said Kim Tae-woo, an expert on North Korea's weapons proliferation at the Korea Institute of Defence Analyses, a think-tank in Seoul.
"North Korea is always going to want the means to maintain its current regime," he told a parliamentary hearing on Tuesday.