"At this time, I am cautiously optimistic about the possibility of resuming the six-party talks in July, but North Korea has not set a firm date for their return," Kim Sook, a South Korean Foreign Ministry official, said on Monday.
Diplomatic hopes were raised for a resumption of the talks after North Korean leader Kim Jong-il told a South Korean envoy last month that Pyongyang could return in July if the United States met certain conditions, such as treating it with respect.
The South's Unification Minister Chung Dong-young, who had the rare meeting with the North Korean leader on 17 June, was in Washington last week to discuss with US officials the talks he had.
Kim Sook joined Chung on that trip, during which the South Korean delegation met Bush administration officials including Vice-President Dick Cheney and Secretary of State Condoleezza Rice.
"It's the right time for the North to return for the talks and commit to a firm date," Kim said.
Pyongyang's refusal to set a firm date has led some analysts to conclude the North is stalling for time. Kim did not specify what gave him grounds for optimism.
But South Korea and the US have agreed to combine proposals to try to convince North Korea to abandon its nuclear weapons programme, Chung was quoted as saying on Saturday.
"It's the right time for the North to return for the talks and commit to a firm date"
Kim Sook, South Korean Foreign Ministry official
Washington has proposed security guarantees and energy aid if Pyongyang gives up its nuclear weapons programme.
Seoul has not given details of its proposal but South Korean newspapers say it goes beyond Washington's.
The newspapers say the idea focuses on a huge injection of aid and technical assistance akin to the US Marshall Plan that was instrumental in putting Western Europe, and particularly West Germany, back on its feet after the second world war.
North Korea said in February it possessed nuclear weapons and was boycotting the talks that include the two Koreas, China, Japan, Russia and the US.