WASHINGTON - Storm clouds are gathering over the Korean Peninsula, Defence Secretary James Mattis has told soldiers and airmen of the elite 82nd Airborne Division at Fort Bragg, North Carolina.

"My fine young soldiers, the only way our diplomats can speak with authority and be believed is if you're ready to go," Mr Mattis told several dozen soldiers and airmen during a two-day tour of US bases, the military journal Stars And Stripes reported on Friday (Dec 22).

During his visit to another US base, the Guantanamo Bay Naval Station, he told the troops "you all have to be ready to go" if diplomacy fails.

The US could not assume that North Korean leader Kim Jong Un does not seek nuclear war, he said.

The Associated Press quoted him saying: "If we have to do it (militarily), we expect to make it the worst day in North Korea's life."

If war comes, "every submarine he's got is to be sunk, and every ship he's got is to be sunk", said the retired general.

At Fort Bragg, Mr Mattis told the troops they should read the book This Kind Of War: A Study In Unpreparedness published in 1963, a decade after the Korean war ended in 1953.

"You know what I'm driving at here," Mr Mattis said as the soldiers listened in silence, the Stars And Stripes reported.

"So you gotta be ready," he added.

The book, a classic history of the Korean War, saw the conflict as a test of wills. The communists, author T.R. Fehrenbach contended, doubted that the United States had the will to react quickly and practically, and without panic.

In the end the US dropped 635,000 tons of explosives on North Korea - more than the amount used in the entire Pacific theatre during World War II. Some 75 per cent of Pyongyang was destroyed, a decisive factor which kept the communists from advancing beyond the 38th parallel.

The three-year war which ended without a peace treaty was perhaps the bloodiest in modern history: The US lost some 37,000 troops, while the total number of Koreans dead, injured or missing, was estimated at close to three million. The majority of the casualties were in the North.

When asked about recent reports that families of American service members in South Korea might be evacuated, Mr Mattis said: "I don't think it's at that point yet".

But he added in comments carried by the Stars And Stripes that there was a contingency plan for getting US service members' families out of South Korea "on very short notice".

The US has around 28,000 military personnel and several tens of thousands of American civilians in South Korea.

The US defence chief's remarks on Friday came on the same day the United Nations Security Council voted to impose more sanctions on North Korea in response to its latest test of an intercontinental ballistic missile (ICBM).

His comments also came two days after the Telegraph newspaper, citing unnamed sources, reported that the US was evaluating a military strike on North Korea to give it a "bloody nose" to show that the US was serious about its threat to use the military option.

The latest round of sanctions target North Korea's energy, export and import sectors. Among other measures, they further lower a cap on refined petroleum supplies from two million barrels to a maximum of 500,000 barrels a year, compared to 4.5 million barrels imported by North Korea last year.

The British newspaper quoted an unnamed former US security official saying: "The Pentagon is trying to find options that would allow them to punch the North Koreans in the nose, get their attention and show that we're serious."

A source who spoke to The Straits Times on condition of anonymity said such a strike was a possibility, especially if North Korea were to conduct another missile test.

The strike, if it happens, could take the form of cruise missiles targeting a North Korean nuclear or missile facility.

The Trump administration was adamant that it would not allow North Korea to further develop its nuclear missiles, the source said.

On Nov 29, Pyongyang tested its most advanced ICBM , the Hwasong-15 missile, which flew the highest and longest any North Korean missile has previously flown. North Korea, which conducted its sixth and most powerful nuclear test in September, says the entire US mainland is now within range of its nuclear arsenal.

Experts say the North Koreans need to conduct more tests to develop the ability to mount a nuclear weapon on the missile and perfect the re-entry technology.

Mr Mattis' comments and the latest round of UN sanctions come as South Korea prepares to host the Winter Olympics in February in Pyeongchang, which is just 80km from the Demilitarised Zone separating the two Koreas.

Analysts are worried that North Korea may attempt more provocations and disrupt the games, pushing what is already a hair-trigger situation to the brink of war.

Analysts and US generals have warned that another war on the Korean peninsula would be catastrophic.

US think tank Brookings in a note this month says: "Kim likely would respond to a limited strike with symmetrical military action, such as artillery strikes against South Korea's islands along the maritime border or shorter-range ballistic missile launches against South Korean or Japanese targets, to show that he will not be intimidated, and to maintain the initiative in driving events on the Korean Peninsula."