South Korea and the United States have agreed to pursue a stand-alone agreement on cooperation for nuclear-powered submarines, with working-level talks expected to begin early next year, National Security Adviser Wi Sung-lac said.

Wi made the remarks after returning from a weeklong trip to the U.S., Canada and Japan, where he held meetings with U.S. Secretary of State Marco Rubio, who also serves as national security adviser, along with U.S. Energy Secretary Chris Wright during his stay in Washington in the US, reports Yonhap news agency.

He said the two sides held "practical and in-depth consultations" on security agreements included in a joint fact sheet released last month following President Lee Jae Myung's summit with U.S. President Donald Trump on the sidelines of the Asia-Pacific Economic Cooperation summit in Gyeongju in late October.

"We shared the view that a stand-alone agreement on cooperation over nuclear-powered submarines is necessary, and agreed to move forward with discussions," Wi told reporters.

During discussions on uranium enrichment and reprocessing of spent nuclear fuel, Wi said he reaffirmed Seoul's commitment to nuclear non-proliferation.

South Korea plans to secure low-enriched uranium at levels below 20 percent to fuel the nuclear-powered submarines, Wi said, stressing that highly enriched uranium is not under consideration.

A working-level U.S. delegation is expected to visit South Korea early next year to hold consultations on implementing the agreements outlined in the joint fact sheet, he added.

Under a bilateral nuclear accord with the U.S., South Korea is largely prohibited from enriching its own uranium and must import all of its low-enriched uranium, said the report.

Meanwhile, HD Hyundai Heavy Industries Co., South Korea's leading shipbuilder, said on Wednesday it has received a 375 billion-won ($256 million) order to build two petrochemical product carriers for a shipper in Africa.

The two vessels will be delivered to the undisclosed African customer by July 2028, the company said in a regulatory filing.