SEOUL (REUTERS) - A South Korean government probe into nuclear safety has uncovered cables with forged safety certificates at two power plants under construction, a source at the sector's regulator said on Wednesday.

The source at the state-run Nuclear Safety and Security Commission said the findings were part of an investigation that began almost two months ago.

South Korea's 23 nuclear reactors generate a third of its electricity needs, and power supplies are tight because three of these reactors are offline after the government probe revealed components with fake documents.

Korea Hydro and Nuclear Power Corp, which is owned by state-run Korea Electric Power Corp, manages all of South Korea's nuclear power plants.

A Korea Hydro spokesman said the company's President and Chief Executive Kim Kyun-seop had tendered his resignation last week, but declined to say whether his decision was linked to the forged parts scandal.

"Our president tendered his resignation last week. We are not sure if the resignation is accepted, and why he submitted the resignation," the spokesman said.

Asked about the nuclear regulator's latest findings, another Korea Hydro spokesman said: "We know the findings of the fake certificates at two additional reactors, although the commission knows the details of the investigation."

Construction at the two reactors was due to finish later this year and in September 2014, Korea Hydro said.

The nuclear problems have increased the risk of power shortages in the hotter than usual Korean summer when power demand is seasonally high for air conditioning. South Korea is likely to import more oil and natural gas to make up for the shortages.