US Defence Secretary James Mattis has told American troops to be prepared for war as the United Nations voted to pass the toughest-ever sanctions on North Korea.

The new UN sanctions, adopted on Friday by a vote of 15-0, seeks to ban nearly 90 per cent of refined petroleum exports to North Korea and demands the repatriation in two years of some 100,000 North Korean labourers who work abroad.

China and Russia joined in the vote, but only after the Trump administration agreed to soften a couple of provisions.

Ms Nikki Haley, the US Ambassador to the UN, thanked the other UN Security Council members - especially China - for coming together on the resolution and said further North Korean defiance would "invite further punishment and isolation".

The sanctions are the latest efforts to force the nuclear-armed country into negotiations over its nuclear programme.

Analysts say the US is fast running out of options. Even the Central Intelligence Agency has said no amount of economic sanctions will force the North to give up its nuclear arsenal.

"If the game plan is to use sanctions as the last non-military policy tool to induce North Korea's return to the denuclearisation table, we may quickly find Washington prioritising military options," Mr John Park, director of the Korea Working Group at Harvard Kennedy School, told Reuters.

During a tour of US bases on Friday, Mr Mattis told soldiers and airmen of the elite 82nd Airborne Division at the Fort Bragg military base in 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."

His comments were carried in the military journal Stars and Stripes.

During his visit to another US base, the Guantanamo Bay Naval Station, Mr Mattis told the soldiers "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, said the retired general.

The Telegraph newspaper, in an exclusive article published on Dec 20, quoted unnamed sources as saying the US is drawing up plans for a "bloody nose" military attack on North Korea.

It quoted an unnamed former US security official briefed on policy as 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 told The Sunday Times that 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 source told The Sunday Times on condition of anonymity.

But such a move would risk sparking a catastrophic war on the Korean peninsula.

North Korea, says US think-tank Brookings in a note this month, is likely to 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 it will not be intimidated, and to maintain the initiative in driving events on the Korean peninsula."

Pyongyang tested its most advanced intercontinental ballistic missile (ICBM) on Nov 29.

Experts say the Hwasong-15 ICBM, with an estimated range of 13,000km, puts the entire US mainland within range. North Korea needs more tests to develop the ability to mount a nuclear weapon on the missile and perfect the re-entry technology.