SEOUL, South Korea — South Korea decided Friday to resume construction of two nuclear plants, reversing a campaign promise by the new president and frustrating supporters who want the country to phase out nuclear power.

In the months before his election, President Moon Jae-in had vowed not to allow any new reactors. When he made that pledge, five nuclear power plants were under construction, three of them near completion. Mr. Moon said that he would scrap the other two, which were both in the early stages.

Since Mr. Moon took office in May, however, the fate of those two reactors has become the center of a raging debate over nuclear power in South Korea, and antinuclear groups were quick to condemn the decision on Friday.

Opponents of nuclear power, who have gained momentum in South Korea since the Fukushima disaster in Japan in 2011 and supported Mr. Moon during his campaign, wanted him to block the two reactors. But conservative media outlets and politicians, as well as the nuclear industry itself, have been just as vocal, insisting that nuclear remained a cheap, clean and reliable source of energy for South Korea, which produces neither crude oil nor natural gas.