Joe Biden's administration put South Korea on the lowest tier of the Sensitive and Other Designated Countries List in January shortly before the then-president left office, the department said in a written response to Reuters' questions on Friday.
The department did not explain why South Korea was added to the list and did not indicate that President Donald Trump was inclined to reverse the measure.
The spokesperson said Seoul faces no new restrictions on bilateral co-operation in science and technology.
South Korea's foreign ministry could not immediately be reached for comment on Saturday.
Seoul is in talks with Washington to resolve the matter before the designation takes effect on April 15, Yonhap news agency reported, citing a diplomatic source it did not identify.
The department list of sensitive countries includes China, Taiwan, Israel, Russia, Iran and North Korea, with Tehran and Pyongyang designated as terrorist, according to a 2017 document posted on the department's website.
President Yoon Suk-yeol and then-defence minister Kim Yong-hyun are among the South Korean officials who raised the prospect that South Korea would be forced to pursue nuclear weapons amid fears over North Korea's accelerated nuclear weapons and ballistic missile programs and concerns about the strength of the alliance with the US, which provides a nuclear shield for South Korea.
Yoon and Kim have been indicted on charges of insurrection over Yoon's six-hour declaration of martial law in December.
Yoon was impeached and his presidential powers suspended while a court decides whether to remove him from office.
Yoon backed off rhetoric about a nuclear weapons program after negotiating with Biden a 2023 agreement under which Washington is to give Seoul more insight into US planning to deter and respond to a nuclear incident in the region.
In return, Seoul renewed a pledge not to pursue a nuclear bomb of its own and said it would abide by the Nuclear Non-proliferation Treaty, which it has signed.
That, however, has not been enough to ease doubts over US defence commitments that have fuelled calls for a South Korean nuclear arsenal.
Foreign Minister Cho Tae-yul said in February nuclear weapons were not "off the table", although it was premature to talk about such a plan.
"Given that international situations are developing in unpredictable directions, this is a principled response that we must prepare for all possible scenarios," Cho told a parliamentary hearing.
Daryl Kimball, executive director of the nonprofit Arms Control Association, said South Korea was a proliferation risk in light of such "provocative" statements and the department was prudent to put the country on its list.
"Listing the ROK as a proliferation-sensitive country should rule out any chance of a South Korean request for US approval to enrich uranium and reprocessing spent fuel ... to produce nuclear weapons," Kimball said, citing the country's formal name, the Republic of Korea.
The handling of the designation raised concerns in Seoul.
Cho told parliament on Tuesday that his ministry had had no formal communication from the Biden administration and had heard about the possible designation from an informal tip-off.
Countries can appear on the energy department's list for reasons of national security, nuclear nonproliferation or support for terrorism, but inclusion does not necessarily indicate an adversarial relationship with the US.
"Currently there are no new restrictions on bilateral science and technology co-operation with the ROK," the department said.
