BEIJING (PTI): China's first indigenously-developed aircraft carrier has begun sea trials, a historic step in the country's quest to modernise its military and bolster its naval presence in the disputed regional waters.

The 50,000-metric tonne vessel left a shipyard of Dalian Shipbuilding Industry in Liaoning province around 7 am (local time) on Sunday against thick fog and with assistance from several tugboats, before navigating into the sea using its own propulsion, state-run China Daily reported.

The vessel will become the country's second aircraft carrier, and the first to be entirely built and designed inside of China, when it joins the navy sometime before 2020.

The Navy said in a statement that the trial aims to demonstrate the reliability and capability of the ship's propulsion systems.

The still-unnamed carrier was launched this time last year but since then has been undergoing fitting of weapons and other systems and has not yet entered service.

China launched the new aircraft after it commissioned the first carrier the Liaoning, a refitted Soviet Union-made vessel in 2012.

Though Liaoning has become operational, it is being mostly used for research and improvements for the new carriers China plans to build.

Reports said China is already building its third aircraft carrier in Shanghai. China reportedly plans to have four aircraft carriers by 2030 to beef up its navy as it asserts its claims in the disputed South China Sea.

Few countries in the world have aircraft carriers. Besides India, the US, Russia, the UK, France, Italy, Thailand and China operate a total of 18 aircraft carriers.

The US is the largest operator as it runs 11 nuclear-powered carriers with each having a full-load displacement of about 100,000 tonnes.

Some reports said China is also planning to build a nuclear aircraft carrier.

Chinese experts have said that the missions for the new carrier will be different from those for the Liaoning, which is mainly tasked with testing equipment and weapons as well as training personnel.

The new vessel, by comparison, will focus on what a genuine aircraft carrier is supposed to do like conducting combat-ready patrols and safeguarding China's maritime sovereignty and interests, the report said.

China has developed a new jet fighter called A J-15 fighter to operate from the decks of its carriers.

The second aircraft carrier's name and hull code remain unknown as the PLA Navy usually makes public such information when a ship is commissioned.

The new carrier has a conventional propulsion system. Like the Liaoning, it will use a ramp to launch J-15 fighter jets. The ship will also carry several types of helicopters.

The new carrier has more than 12,000 pieces of equipment that were made by 532 Chinese enterprises including many private firms, the report said.

The vessel has more than 3,600 cabins. About 3,000 workers from across the country worked on the ship on a daily basis.

Officials of the Navy said, several aspects of the new carrier are different from those on the Liaoning. For example, the new ship's island is shorter and the landing section is longer.