A senior US diplomat said Wednesday that the US government is carrying out a "pretty extensive process" regarding a bilateral agreement with South Korea over the Asian ally's push to acquire nuclear-powered submarines.

Under Secretary of State for Arms Control and International Security Thomas DiNanno made the remarks during a hearing of the House Foreign Affairs Committee, commenting on the submarine project included in a joint fact sheet that Seoul and Washington issued in November to outline their agreements on security, trade and other issues.

Under Secretary of State for Arms Control and International Security Thomas DiNanno (Yonhap)
Under Secretary of State for Arms Control and International Security Thomas DiNanno (Yonhap)

In the fact sheet, the two governments codified the US' approval for South Korea to build nuclear-powered attack submarines, noting that Washington will work closely with the Asian ally to advance requirements for the shipbuilding project, including "avenues to source fuel."

"Specific to a nuclear submarine as part of the joint fact sheet, we're going through a pretty extensive process," the official said. "Interagency is very active on building teams who would go, what our deliverable, deliverables would be, because they're extensive."

DiNanno noted that the process is proceeding in a collaborative manner with the Pentagon working on the infrastructure dimension and the State Department on the issues related to any potential nuclear fuel provision for the submarine project.

The official also reaffirmed the US "extended deterrence" commitment to South Korea.

"The president himself on the (Korean) peninsula was very clear in his commitment to the US extended deterrence umbrella. So there can be no clearer message," he said, referring to President Donald Trump.

Extended deterrence refers to the US commitment to using the full range of its military capabilities, including nuclear weapons, to defend its ally. (Yonhap)