US and Chinese sailors faced off on Chinese soil Tuesday, in sporting clashes between their giant militaries aimed at building trust against a backdrop of tensions over Beijing's assertion of its territorial claims. The contests -- the hosts claimed a 3-1 victory at football, but the visitors dominated on the basketball court -- came as the USS Blue Ridge, the flagship of the US Navy's 7th Fleet, paid a rare visit to the base of China's South Sea Fleet. Its frigates are charged with defending Beijing's claim to almost all the South China Sea, against several Southeast Asian neighbours. Chinese officers toured the hulking visiting command ship, docked beside People's Liberation Army vessels in Zhanjiang in the southern province of Guangdong, and later eyed up their US counterparts over lunch. On the Blue Ridge's main deck, US personnel patrolled with M-16 rifles a stones' throw from the palm-fringed Chinese shore, while on land staff from both navies swapped jokes and rebounds. The Chinese and US militaries -- the world's largest and most powerful respectively -- have been increasing exchanges even as Beijing's assertion of its South China Sea claims, most recently through rapid building of artificial islands, rings alarm bells in Washington. US Admiral Harry Harris, commander of the US Pacific Fleet of which the 7th Fleet is a part, said Tuesday: "China is responsible for the rise of tensions and provocations in the South China Sea." "I don't think that there's a likelihood of a major force-on-force conflict in the South China Sea today," he added while visiting the Indonesian capital Jakarta. "But I have to be ready for that." - Dangerous ambiguity - The South China Sea is a vital strategic waterway and close encounters between the two powers' ships have led to fears of a clash. In their most serious maritime incident for years, the US guided missile warship Cowpens had to make a sharp turn to avoid colliding with a Chinese naval ship that cut in front of it in the South China Sea in 2013, according to the Pentagon. The two navies "interact, often daily. So building this relationship is key", Lt. Charles Banks of the 7th Fleet told AFP, adding that naval officials on both sides would discuss the implementation of a "Code for Unplanned Encounters at Sea" during the five-day visit. "Ambiguity can lead to confusion and mistakes," he said, as US soldiers in black vests and day-glo trainers teamed up with Chinese sailors. "It's to build good relations," said Terrence Phillips, a boatswain's mate on the Blue Ridge and a centre on the basketball court. "Sport is about international camaraderie for everybody." A female Chinese Navy engineer, who said she was not authorised to give her name, ran to take a photo with Phillips and said: "When we play basketball it's very friendly." The US Navy still eclipses China's fleet in terms of firepower -- Washington currently has 10 aircraft carriers to Beijing's one. An 11th carrier, the Gerald R. Ford, is scheduled for delivery next year, according to the US Navy's website. Chinese media, meanwhile, have quoted officials as saying a second carrier is under construction. China has had double-digit military budget growth and President Xi Jinping vows to build a "strong navy" capable of "fighting and winning battles". The base in Zhanjiang was lined with posters repeating Xi's call. "The South China Sea has become a greater source of tension in the US-China relationship since 2010," said M. Taylor Fravel, a professor at the Massachusetts Institute of Technology. At the same time, the two sides have stepped up military exchanges, which were set to continue Tuesday evening at a drinks reception on board the Blue Ridge. "The fact that these exchanges are occurring faster and more broadly than before underscores that the US-China relationship has both elements of cooperation and competition," Fravel added.