China's first aircraft carrier has returned to port after completing a "smooth" set of sea trials designed to test its capabilities, the state news agency Xinhua reported on Monday. The 300-metre (990-foot) ship docked in the northeastern port of Dalian on Sunday morning after five days of trials that have sparked international concern about the country's widening naval reach. The carrier tests came amid heightened tensions over a number of maritime territorial disputes involving China, notably in the South China Sea, which is believed to be rich in oil and gas and is claimed by several countries. Dockworkers set off fireworks as the vessel, a refitted old Soviet carrier called the Varyag, returned to port, Xinhua said, adding that the ship would now undergo further work and testing. "The sea trials carried out by the aircraft carrier on its maiden voyage went smoothly," it said. China's People's Liberation Army -- the world's largest active military -- is extremely secretive about its defence programmes, which benefit from a huge and expanding military budget boosted by the nation's runaway economic growth. Earlier this year, China announced military spending would rise 12.7 percent to 601.1 billion yuan ($91.7 billion) in 2011. In January it revealed it was developing its first stealth fighter jet. It is also working on an anti-ballistic missile capable of piercing the defences of even the most sturdy US naval ships. Japan recently expressed concern about what it called the "opaqueness" of China's military budget, and the US State Department last week called on the country to explain why it needed an aircraft carrier. "This is part of our larger concern that China is not as transparent as other countries," said spokeswoman Victoria Nuland. "It's not as transparent as the United States about its military acquisitions, about its military budget." Beijing only recently confirmed it was revamping the old Soviet ship. It has repeatedly insisted that the carrier poses no threat to its neighbours and will be used mainly for training and research purposes. But a news website run by China's defence ministry took a different stance four days ago, stating that the carrier should handle territorial disputes as well.