South Korea is considering boosting its three-axis defense program by building its own nuclear-powered submarines and acquiring spy satellites to counter the North Korea threat.

At their second bilateral meeting on Thursday, US President Donald Trump and South Korean President Moon Jae-in committed to strengthen their combined defense posture against the People's Democratic Republic of Korea (DPRK) through Seoul's acquisition and development of highly-advanced military assets, as well as through the deployment of US strategic assets in and around South Korea.

Following the meeting, Trump said it was "a great privilege" to talk to Moon and that the ongoing North Korean crisis was the most important issue he and his counterpart had to address.

"I am allowing Japan and South Korea to buy a substantially increased amount of highly sophisticated military equipment from the United States," Trump tweeted on September 5, after Pyongyang successfully tested a hydrogen bomb that could be loaded on an intercontinental ballistic missile.

During his presidential campaign, President Moon said that South Korea needed nuclear-powered submarines "in this era," pledging to make efforts to revise a nuclear cooperation deal with the US.

Prior to his meeting with Trump, the JoongAng Ilbo cited senior South Korean government officials as saying that Seoul and Washington have agreed in principle to let South Korea build nuclear-powered submarines. The Blue House dismissed the reports, however, saying that no agreement has yet been reached.

On Friday, a senior South Korean defense ministry official told YonhapNews that rather than buying such subs from the US, South Korea would acquire them "in the form of indigenous development." Experts agreed that South Korea had the capability.

"South Korea can build its own nuclear-powered submarines if it makes the SMART reactor, developed by the Korea Atomic Energy Research Institute, smaller," Moon Keun-shik, a retired South Korean Navy captain, told YonhapNews. "In that case, South Korea will be able to launch its own nuclear subs within five years."

But to launch its own nuclear subs, Seoul will have to meet the cost of at least 1 trillion won ($880 million) for each, plus cover enormous annual maintenance costs, and secure a stable supply of nuclear fuel.

The South Korean government also reportedly plans to spend more money on surveillance assets including spy satellites and drones. Currently its military is dependent on the US for satellite information on DPRK missile bases and nuclear test sites.

South Korea's "three axis defense platform" strategy was first unveiled in late 2016 and involves three critical elements; The Kill Chain pre-emptive strike, the Korean Air and Missile Defense (KAMD), and the Korea Massive Punishment and Retaliation (KMPR) scheme.