SEOUL (REUTERS) - South Korea's nuclear power plant operator said on Tuesday it was investigating a new threat posted on a Twitter account that a fresh batch of data had been stolen from the agency.

The Twitter account had previously been used to claim the theft of data last week.

The user, self-described as the chairman of an anti-nuclear group, demanded the shut down of some of the country's 23 nuclear reactors and urged local residents to flee the areas in a post that included a few sets of data.

"We have not yet opened the newly posted files," an official at Korea Hydro and Nuclear Power Co Ltd (KHNP) said by telephone, citing concerns about the safety of the data. "It will take some time to verify the data they posted."

The KHNP said on Monday that its computer systems had been hacked but only non-critical data had been stolen. Operations were not at risk, it said.