North Korea's relationship with China has been historically described as akin to "lips and teeth", in that the nations cannot function without each other.

Key points: China is also North Korea's largest trading partner and supplies aid, oil and fuel

China is also North Korea's largest trading partner and supplies aid, oil and fuel The relationship between the two nations took a hit when North Korea started nuclear testing

The relationship between the two nations took a hit when North Korea started nuclear testing Mr Kim travelled to Hanoi for the summit via China, a move analysts said was "symbolic"

The Chinese saying refers to the country's 70-year relationship dating back to the Korean War of 1950-1953, when the Soviet Union and China supported North Korea as its army invaded South Korea.

Since the collapse of the Soviet Union, China has been described by analysts as "the most important country sustaining North Korea", but the historic "blood alliance" — a term used to describe their relationship dating back to North Korean supreme leader Kim Il-sung, and Chairman Mao Zedong — has been far more complicated.

On one hand, North Korea has relied on China for aid, oil and fuel imports since the United Nations imposed trade sanctions in 2006, while China is also North Korea's largest trading partner.

On the other, North Korea provides a crucial buffer between China and its adversary, US-backed South Korea.

North Korea, the US and China signed an armistice agreement in 1953, but South Korea refused. ( Wikimedia Commons )

Nick Bisley, head of the school of humanities and social sciences at La Trobe University, said the origin of the partnership between the countries started because of their shared communist ideology, but that their relationship has since developed.

Dr Bisley previously explained to the ABC that the Chinese often used the phrase "lips and teeth" in public, with North Korea being the "lips" and China being the "teeth".

"The teeth are protected by the lips. But also — and this is the North Korean perspective — the teeth can bite the lips," he told the ABC.

Dr Bisley said the relationship between the two countries had become strained in the past three to four years because North Korea had "sprinted to the finish line in its nuclear acquisition".

He was referring to Pyongyang's internationally condemned series of nuclear tests — which became the catalysts for the US-North Korea summits.

"I think the Kim regime was essentially realising that there wasn't much that China could do to bring North Korea to heel," Dr Bisley said.

"There was a period that Kim didn't travel to Beijing in three years, which is unheard of for a North Korean leader in an environment in which they're so economically dependent on [China]."

'Worst case scenario' for China

Kim Jong-un's decision to travel through China to Hanoi was symbolic, experts say. ( KCNA via Reuters )

Leszek Buszynski, visiting fellow at the Strategic and Defence Studies Centre at the Australian National University, told the ABC that ensuring North Korea's survival and keeping them as an ally was "crucial".

He said stopping all exports to North Korea could lead to the collapse of the regime, which would have a disastrous impact on China in many ways.

This could include a humanitarian crisis with an influx of North Korean refugees crossing the border into China.

"China has supported North Korea, no matter how exasperated the Chinese leadership has become," he said.

If the North Korean regime collapsed there would be fears that the leadership wouldn't survive, and reunification of the Korean Peninsula would be on the terms of the US-allied South, Dr Buszynski said.

He added the Sino-North Korea relationship had become more "complex" as North Korea sought to improve its diplomatic relations with both South Korea and the United States.

Beijing "do not want reunification at the moment", he added.

Dr Bisley believes the "worst case scenario" for China was a reunited Korean Peninsula, however, both analysts agree that it is a long way off.

"It's China's worst nightmare, that you have a Republic of Korea, a US ally at your doorstep," he said.

China's invisible presence during Hanoi summit

Sorry, this video has expired Donald Trump and Kim Jong-un discuss setting up a US liaison office in Pyongyang

During a press conference after the US-North Korea summit in Hanoi on Thursday, US President Donald Trump said China had been "a big help" in its dealings with North Korea.

Analysts also said the invisible presence of China was significant.

"China wants to maintain its position as some kind of balancer of major players on the Korean Peninsula, through its very close relationship with the North," Dr Buszynski said.

"That will continue. The Chinese won't give that up."

Dr Bisley added that the symbolism of Mr Kim travelling to Hanoi for the summit on a train that passed through China clearly showed the relationship was on the mend.

"It shows [Mr] Kim will check in with [his] main ally before making any major moves with the US," he said.

"There's a sort of symbolic weight through all [of] that — so you could say the relationship is again like lips and teeth."