Russian delegation head Alexander Alexeyev said on Thursday the Korean Peninsula nuclear issue is unlikely to be solved within a short period of time as it has been an issue for years.

"The problems have accumulated over the years, no matter how wishful you are or how serious the problem is, it could not be solved within three or five days," Alexeyev told a press briefing on the six-party talks under way.

The delegations of the United States and the Democratic People's Republic of Korea (DPRK) held three-hour one-on-one consultations on Thursday morning and agreed to continue consultations.

Describing the U.S.-DPRK consultations as difficult, Alexeyev said "it is the first time for both sides to speak so deeply," adding that "they are tough, but they could be more flexible."

According to Alexeyev, the fourth round of six-party talks is not the final round.

Alexeyev, also Russian deputy foreign minister, said he would leave Beijing for Moscow on Saturday. But he added that his deputywould remain in Beijing and he would come back "as soon as it's necessary to come back,"

Some members of the other delegations will also return home to consult with their respective governments, he said.

The fourth round of the six-party talks, involving China, the DPRK, the United States, the Republic of Korea (ROK), Russia and Japan, began on Tuesday in the Chinese capital.

A major task for Russia and other parties is to push forward the process of the six-party talks, and to work together with the United States and DPRK to find "reasonable and fair" solutions to the nuclear issue, Alexeyev said.