The vessel is part of a submarine programme launched in 2016 that aims to deliver a fleet of eight vessels.

China claims Taiwan as part of its territory and has threatened to use force to annex the self-ruled island, which insists it is a sovereign nation and not subordinate to Beijing.

Shipbuilder CSBC Corp said Tuesday that sea trials of the submarine began in the southern port of Kaohsiung on Saturday, nearly two years after the vessel was unveiled.

The submarine successfully wrapped up the trial -- the first of three stages -- on Tuesday with a "floating navigation test", the company said in a statement.

"CSBC Corporation will continue, based on the test results, to adjust and improve each system, and proceed to the next phase of submerged navigation tests," the company said.

The submarine measures 80 metres (262 feet) in length, has a displacement weight of about 2,500 to 3,000 tons, and boasts combat systems and torpedoes sourced from the US defence company Lockheed Martin.

The aim is to finish sea trials by September 30 and deliver the submarine by the end of November, the Navy said last month.

But the submarine programme has faced obstacles.

The main opposition Kuomintang party and the Taiwan People's Party, which together control the parliament, froze part of the programme's budget earlier this year.

The parties said they wanted to see the results of the submarine's sea trials before releasing the funds.

Taiwan, which would be massively outgunned by China in a war, is trying to boost its ability to wage "asymmetric warfare" involving more agile equipment like submarines and drones.

But there are concerns that opposition efforts to cut military spending could hurt the island's ability to strengthen its firepower.

Taiwan's navy currently has two working submarines, Swordfish-class vessels bought from the Netherlands in the 1980s.

Over the same period, China has built itself one of the world's largest navies, with nuclear-powered submarines and aircraft carriers.