WASHINGTON (REUTERS) - President Donald Trump, like his predecessors, may find that neither negotiations nor economic and military pressure can force North Korea to abandon its nuclear programme, and that the United States has no choice but to try to contain it and deter North Korean leader Kim Jong Un from ever using a nuclear weapon.

North Korea conducted its sixth and most powerful nuclear test on Sept 2, describing it as an advanced hydrogen bomb for a long-range missile, a dramatic escalation of its stand-off with the United States and its allies.

US officials declined to discuss operational planning, but acknowledge that no existing plan for a preemptive strike could promise to prevent a brutal counterattack by North Korea, which has thousands of artillery pieces and rockets trained on Seoul.

In an implicit recognition that the military options against the North are unpalatable at best and pyrrhic at worst, US Defence Secretary Jim Mattis last week told reporters: "We are never out of diplomatic solutions."

US and Asian officials believe it is necessary to try negotiations and more economic pressure but concede these are unlikely to curb, let alone eliminate, the nuclear and missile programs that North Korean considers essential to its survival.

That leaves Washington and its allies in South Korea, Japan and elsewhere with an unwelcome question: Is there any way to live with a nuclear-armed North Korea, one that is contained and deterred from using its nuclear weaponry?

Trump declined to answer that question at a news conference on Thursday, saying he would not disclose his negotiating strategy publicly and adding it would be a "very sad day" for North Korea if the US military settles the matter.

"Military action would certainly be an option. Is it inevitable? Nothing is inevitable," Trump said.

STRENGTHENING DETERRENCE

The only alternative, the current and former officials said, speaking on the condition of anonymity, is some combination of containment and deterrence similar to the threat of Mutual Assured Destruction and arms control treaties that helped prevent a nuclear war between the United States and the Soviet Union.

Among the US options to strengthen its deterrent is the long-planned modernisation of America's aging nuclear forces that would assure that North Korea would be destroyed if it fired a nuclear-tipped missile at the United States, a US military base, Japan, or South Korea.

Another is stepped-up investment in US missile defences, particularly testing, research and development of technologies that could defeat a significant number of incoming missiles.

Both steps would need to avoid triggering new arms races with Beijing and Moscow, experts say.

Another factor weighing on Pentagon planners is their readiness for a major conventional conflict after 16 years of war in Afghanistan, Iraq, Syria and elsewhere.

There has been no sign the White House, which has been cool to the idea of talks and hopes pressure can change the North's calculus, is ready to settle for a containment strategy.

Despite pessimism about talks, a US official speaking on condition of anonymity said there was a chance that economic pressure, especially from China, combined with an agreement to negotiate could convince Pyongyang to limit its nuclear arsenal or even sign the 1996 Comprehensive Nuclear Test-Ban Treaty.

"Signing the CTBT would given the North tacit admission to the nuclear club but end its testing programme," the official said. "That, along with assured destruction, might be the best that could be done."

The remaining question, however, is whether Trump would be willing to settle for that.

"Discipline and steadiness are not words one usually uses in a sentence that also has the name Donald Trump," said Robert Einhorn, a former State Department official who negotiated with North Korea and is now at the Brookings Institution think tank.

"Would he over time recognise that he may have no choice?"

Frank Jannuzi, president of the Mansfield Foundation, which promotes US-Asia relations, is more optimistic.

"Does he have the patience to manage a difficult process of deterrence and containment against the (North) rather than doing something impulsive? I think so," he said.

"Some of his deals have taken years to come to fruition."