North Korean leader Kim Jong Un said the country is now implementing a nuclear force construction policy to increase the number of nuclear weapons "exponentially," according to state media.

In a speech on North Korea's founding anniversary on Monday, Mr Kim said the country must more thoroughly prepare its "nuclear capability and its readiness to use it properly at any given time in ensuring the security rights of the state", said KCNA.

A strong military presence is needed to face "the various threats posed by the United States and its followers," he added.

Mr Kim also said North Korea is facing a "grave threat" from what it said was a US-led nuclear-based military bloc in the region.

The leader said North Korea will "redouble its measures and efforts to make all the armed forces of the state including the nuclear force fully ready for combat," KCNA said.

While Mr Kim has repeatedly made similar pledges, his latest threat comes as outside experts believe he will perform provocative weapons tests ahead of the US presidential election in November.

North Korea, as of Tuesday morning, did not appear to have staged any major military demonstration to mark the 76th founding anniversary.

But the North's main Rodong Sinmun newspaper on Sunday published a photo of Mr Kim inspecting what appeared to be a 12-axle missile launch vehicle, which would be the largest the country has shown so far, during a visit to a munitions plant.

When asked about the photo on Monday, Pentagon spokesperson Major General Pat Ryder refused to provide a specific assessment of North Korea's missile capabilities and reiterated that Washington was working closely with Seoul, Tokyo and other partners to preserve regional security and deter potential attacks.

"It's not unusual for North Korea to use media reports and imagery to try to telegraph, you know, to the world," he said.

In recent days, North Korea has also resumed launches of trash-carrying balloons toward South Korea.

North Korea has been protesting the July signing of a new US-South Korean defence guideline meant to integrate US nuclear weapons and South Korean conventional weapons to cope with growing North Korean nuclear threats.

North Korea said the guideline revealed its adversaries' plots to invade the country. US and South Korean officials have repeatedly said they don't intend to attack the country.

South Korea will hold a defence ministerial meeting with the member states of the United Nations Command (UNC) on Tuesday.

The UNC is led by the commander of the US military stationed in South Korea.

Last month, Germany became the latest to join the UNC in South Korea that helps police the heavily fortified border with North Korea and has committed to defend the South in the event of a war.

North Korea has criticised the UNC as an "illegal war organisation" and Germany's entry into the US-led UN border monitoring force as raising tensions.

Many analysts believe North Korea has some last remaining technological barriers to overcome to acquire long-range nuclear missiles that can reach the US mainland, though it likely already possesses missiles that can hit key targets in South Korea and Japan.

Reuters/AP