"Given the DPRK's recent aggressive nuclear policy and advancements in nuclear capabilities, the... scenario focused on the possibility of the DPRK's use of nuclear weapons," a joint statement said, referring to North Korea.

The US and South Korean delegations "focused their discussion on alliance deterrence to maintain peace and stability on the Korean Peninsula and potential options for responding to DPRK nuclear weapons use," the statement said.

"Both sides affirmed that the alliance stands ready to respond to the DPRK's nuclear threats."

The delegations -- which were made up of military and defense officials as well as personnel from South Korea's foreign ministry and the US State Department -- held the exercise on Wednesday, the same day as separate drills by Washington, Seoul and Tokyo in the Sea of Japan.

Those drills were held with an emphasis on "sharing ballistic missile target information and mastering detection, tracking, and interception procedures," Seoul's defense ministry said in a statement.

On Thursday, the US and South Korean delegations visited a submarine base in Georgia.

During the visit, a senior US Navy officer "explained the mission of Ohio-class nuclear-powered ballistic missile submarine (SSBNs), and emphasized that SSBN forces operated by the US are a key means of providing US extended deterrence to Allies," the statement said.

The United States has sought to reassure South Korea over America's so-called extended deterrence commitments, after North Korea declared itself an "irreversible" nuclear power last year and has conducted a series of banned weapons tests, including multiple missile launches in recent days.

On Thursday the North test-fired four strategic cruise missiles into the sea, state media said, adding that the drill demonstrated the conflict readiness of Pyongyang's "nuclear combat force."