On a grey stone column in Pyongyang, a mural shows Chinese and North Korean soldiers rushing into battle against US-led forces in the Korean War. Decades later, the monument is a regular stop for new waves of Chinese going to the North, this time as tourists.

Hundreds of soldiers and workers have been sprucing up the obelisk and its grounds in recent days ahead of a state visit to Pyongyang by Chinese President Xi Jinping this week.

An inscription on it lauds "the Chinese People's Volunteer Army, who fought with us on this land and smashed down the common enemy".

Their "immortal exploits" will "last forever", it proclaims, as will "the friendship forged in blood between the peoples of the People's Republic of China and the Democratic People's Republic of Korea".

Nearly 70 years after Mao Zedong sent millions of soldiers to save Kim Il Sung's troops from defeat as General Douglas MacArthur's men marched up the peninsula, China remains the isolated, nuclear-armed North's key diplomatic backer and main provider of trade and aid.

Now the Friendship Tower, as the monument is known, attracts growing hordes of Chinese tourists -- and the renovations suggest it may also be on Xi's itinerary.

Ordinary Chinese pay travel companies around 2,500 yuan (S$495) for a standard three-day trip, arriving overland by train in Pyongyang to tour the capital's highlights, from the Arch of Triumph to Kim Il Sung Square.

The following day they head south to the Demilitarized Zone that has divided the peninsula since the two sides fought each other to a stalemate in 1953, before returning home.

"I'm very interested in North Korea and wanted to come to see what North Korea looks like," said Yu Zhi, a retiree from Anhui province visiting Pyongyang, telling AFP that she had a "special feeling" for the country.

"China is very friendly with North Korea," added her fellow traveller, a woman surnamed Jin. "We have been friends for generations."

LIPS AND TEETH

It was not always so. Mao -- whose eldest son Mao Anying was among those killed in what China still calls the "War to Resist US Aggression and Aid the DPRK" -- described the neighbours as "as close as lips and teeth".

Ties then waxed and waned during the Cold War, when founder Kim Il Sung was adept at playing his Soviet and Chinese allies off against each other, and his grandson, the current leader Kim Jong Un, did not visit Beijing to pay his respects for more than six years after inheriting power.

But as he embarked on a flurry of diplomacy last year he made sure that Chinese President Xi Jinping was the first foreign head of state he met, and he has since done so three more times -- more often than Kim has seen any other leader.

Now Xi is going to reciprocate.

At the same time Chinese tourism to the North has reached record highs, according to travel industry sources -- so much so that Pyongyang has imposed a limit on arrivals.

No official figures are available from authorities on either side, but Simon Cockerell, general manager of Koryo Tours, the market leader for Western visitors, said there had been "a huge increase in Chinese tourists".

At peak times 2,000 people a day had been arriving in Pyongyang, he said. "That's far too many because there is no infrastructure to accommodate that many tourists, so problems with train tickets, with plane tickets, hotel space."

As a result North Korean authorities had themselves set a 1,000-a-day cap, he added, although it was unclear whether this applied across the industry or solely to Chinese, who make up the vast majority of arrivals.

"There are issues with just hundreds of people showing up at the same time."

'CHOICES BEING MADE'

China has a proven willingness to use tourism as a geopolitical negotiating weapon -- it banned group tours to South Korea after it deployed a US anti-missile system, THAAD.

With nuclear negotiations at a stalemate the North remains subject to multiple UN Security Council sanctions, and the US imposed a travel ban on its own citizens visiting following the death of student Otto Warmbier, who had been jailed after trying to steal a propaganda poster.

But tourism is not among the sectors targeted by the UN, potentially enabling Beijing to use it as an incentive for its sometimes wayward ally.

The Chinese travel phenomenon is market-driven, rather than prompted by state order -- as well as the market offered by China's huge population, the two countries' border enables cheap overland journeys.

But simply enabling it to take place, said John Delury of Yonsei University in Seoul, meant "We can infer some choices are being made" by Beijing.

"We know it's a lever they can turn on and off," he said.

Even with the diplomatic process at a standstill, he added, "The Chinese think you have to use this window of opportunity to move things forward. There has to be a path on both sides and so something like opening up tourism is a good way to enable that."

At the Monument to the Three Charters for Reunification on the edge of Pyongyang, where two giant stone women form an arch over a road, a secondary school teacher from Shanghai called Peng said: "We are both socialist countries. I feel there are more Chinese coming to visit."