US Defence Secretary Jim Mattis says North Korea is posing an increasing threat to Seoul and the world through its "illegal and unnecessary" missile program.

Key points: Jim Mattis says the US will never accept a nuclear North

Jim Mattis says the US will never accept a nuclear North The CIA says North Korea could be only months away from a nuclear attack on the US

The CIA says North Korea could be only months away from a nuclear attack on the US Donald Trump will make his first presidential visit to Asia next week

Speaking in Seoul with South Korean Defence Minister Song Young-moo, General Mattis said the US's military and diplomatic cooperation with the South had taken on a new urgency.

"North Korea has accelerated the threat that it poses to its neighbours and the world through its illegal and unnecessary missile and nuclear weapons programs," he said.

"I cannot imagine a condition under which the United States would accept North Korea as a nuclear power."

General Mattis had been at pains during his week-long trip to Asia to stress that diplomacy was the preferred course for the US, a message he returned to after top-level military talks in South Korea.

He did clarify diplomacy was most effective "when backed by credible military force".

"Make no mistake, any attack on the United States, or our allies, will be defeated. And any use of nuclear weapons by the North will be met with a massive military response that is both effective and overwhelming," he Mattis said.

Tensions between North Korea and the US have been building after a series of nuclear and missile tests by Pyongyang and bellicose verbal exchanges between Kim Jong-un and Donald Trump.

Earlier this week, Japan warned the hermit state's nuclear and ballistic missile capabilities had grown to an "unprecedented, critical and imminent" level.

The CIA has said North Korea could be only months away from developing the ability to hit the US with nuclear weapons, a scenario Mr Trump has vowed to prevent.

Mr Song told the news conference he and Mr Mattis agreed limits on South Korea's conventional missile warhead payloads would be lifted. He offered no specifics.

Sorry, this video has expired North Korea has launched a series of missiles in recent months.

Also discussed were the conditions under which South Korea would be given wartime operational control of its forces.

Currently, if war with the North broke out, the South's forces would operate under US-led United Nations Command.

Mr Mattis said the North was, in effect, shooting itself in the foot.

"If it remains on its current path of ballistic missiles and atomic bombs, it will be counterproductive, in effect reducing its security," he said.

This was Mr Mattis' second visit to South Korea since taking office in January.

Mr Trump — who has threatened to "totally destroy" the North if necessary — will be on his first trip as President to Asia next week, including a stop in South Korea to meet President Moon Jae-in.

Mr Moon, after talks with Mr Mattis on Friday, said the "aggressive deployment" of US strategic assets in the region, which have included overflights by US bombers, had been effective in deterring the North Korean threat.

Sorry, this video has expired Secretary of State Rex Tillerson says US-North Korea diplomacy will continue until 'first bomb drops'

AP/Reuters