By Jon Herskovitz

SEOUL, Aug 3 (Reuters) - The South Korean government has told Taliban insurgents holding 21 Koreans there is a limit to what it can do to resolve the hostage stand-off that has stretched into a third week, an official said on Friday.

There has been some contact with the Taliban and a South Korean delegation arrived on Thursday in the Afghan province where the Koreans are held hostage to try to hold direct talks with the kidnappers.

"Through our contacts, our foremost goal is to make it clear that there is a limit as to what our government can do to meet their demands of releasing the prisoners," presidential spokesman Chun Ho-sun told reporters.

The Taliban have killed two of their male hostages, accusing the Afghan government of not negotiating in good faith and ignoring their demand to release rebel prisoners. The remaining hostages include 18 women.

The Taliban have repeatedly threatened to kill the rest if their demands are not met.

Separately, eight South Korean lawmakers met State Department officials in Washington on Thursday to seek help.

"We have confirmed the complete support and sympathy for the Korean hostages who are going through great distress," lawmaker Park Jin told reporters after the meeting.

The South Korean government has called for "flexibility", a comment analysts say is directed at the United States to sway the Afghan government to strike a deal with the kidnappers.

There have been calls among many left-leaning politicians for the United States to use its influence to resolve the issue but Washington has stood firm in its refusal to make concessions with groups, such as the Taliban, it considers terrorists.

Others in South Korea have warned such pressure could strain ties.

"We assess the United States is actively cooperating by all its means as best as it can. This is not a matter that should lead to anti-U.S. problems," the presidential spokesman said. (With additional reporting by Jessica Kim in Seoul and Paul Eckert in Washington)

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