South Korea defends the “aggressive deployment” of US strategic assets on the Korean Peninsula as Pentagon chief James Mattis visits the heavily fortified border between the North and South Korea.

President Moon Jae-in said Friday that the US deployment has been effective in deterring North Korea which has stepped up its nuclear and ballistic missile tests in the face of harsh rhetoric from US President Donald Trump.

Standing inside the Demilitarized Zone (DMZ) that separates the two Koreas, Mattis said there is no change in US policy towards North Korea which he accused of building a nuclear arsenal to "threaten others with catastrophe."

"We stand shoulder to shoulder with you and the Korean people in confronting the threats posed by the Kim Jong-un regime," Mattis said alongside his South Korean counterpart Song Young-moo.

Nevertheless, the US remains committed to a "diplomatic solution," he said, adding “our goal is not war but rather the complete, verifiable and irreversible denuclearization of the Korean Peninsula."

Mattis has already said the US is prepared to take military action if the North does not halt its development of missiles that could strike the entirety of the United States, potentially with a nuclear warhead.

The Pentagon chief is set to hold an annual military meeting in Seoul on Saturday during his two-day trip, which comes ahead of a planned trip by Trump to South Korea next month.

Separately, North Korea said it will release on Friday a South Korean fishing boat that was found to be in North Korean waters illegally.

North Korea “decided to repatriate the boat and its crew from the humanitarian point of view, taking into account the fact that all the crewmen honestly admitted their offense, repeatedly apologizing and asking for leniency,” the North’s official Korean Central News Agency (KCNA) said Friday.

The US and South Korea conducted a five-day naval drill, which began October 16, and involved the nuclear aircraft carrier Ronald Reagan, three nuclear submarines, Aegis destroyers and more than 40 other battleships and fighter aircraft “of all kinds.”

North Korea warned that the drills have increased tensions on the Korean Peninsula.

Joint US military exercises with forces from South Korea and Japan have been a constant cause of tensions with North Korea, which have skyrocketed in recent weeks. World leaders have called for restraint by all sides to the conflict.

There has also been a flurry of US military hardware movement around the Korean Peninsula in recent weeks.

Earlier this month, the nuclear-powered USS Michigan submarine arrived at South Korea’s southern port of Busan, according to Yonhap news agency, just days after another nuclear-powered submarine — the USS Tuscon — left after a five-day visit.

Also this month, the US flew two supersonic heavy bombers over the Korean Peninsula. That mission came 17 days after four US F-35B stealth fighter jets and two B-1Bs flew over the peninsula.

Trump has engaged in an increasingly escalating war of words with North Korean leader Kim Jong-un, trading insults amid rising tensions between the two nuclear-armed countries.

Trump recently declared that the US would “totally destroy” North Korea if necessary. North Korea has repeatedly announced that its missile and nuclear development programs were of a defensive nature.

Tensions between Washington and Pyongyang have escalated in recent months, with both countries threatening each other with a nuclear strike.

A CNN poll released this month found that Americans are growing more concerned about how Trump is handling the escalating situation. More than six in 10 say Trump's responses to North Korea's threats have been more reckless than responsible.

An NBC News/SurveyMonkey survey released Wednesday showed that 72 percent of surveyed Americans fear the US will get involved in a major war, possibly with North Korea, during Trump’s first term in office.