BEIJING (Reuters) - The carrier group led by China’s first aircraft carrier, the Liaoning, has reached “initial” combat readiness, the defense ministry said on Thursday, in another significant step in the country’s ambitious military modernization program.

Little is known about the aircraft carrier program, which China regards as a state secret.

It is part of President Xi Jinping’s sweeping plan to refurbish the armed forces by developing everything from stealth jets to anti-satellite missiles, as China ramps up its presence in the disputed South China Sea and around self-ruled Taiwan.

China bought the Soviet-era Liaoning secondhand from Ukraine in 1998, before refitting mainly for training use as it honed the ability to operate fighter jets at sea and with other warships.

But the Liaoning has gone on increasingly high-profile missions recently, such as sailing around Taiwan, which China claims, and into the South China Sea.

The Liaoning and its accompanying carrier group had successfully been carrying out training missions, Defense Ministry spokesman Ren Guoqiang told a monthly news briefing.

“The carrier group’s exercises have been deepened to include combat operations in the open seas. It has initially formed a system combat capability,” Ren said, without elaborating.

China’s second, domestically-developed, carrier, began sea trials earlier in May. It is as yet unnamed.

Chinese military experts have told state media the new carrier, built in the northeastern port of Dalian, is not expected to enter service until 2020, once it has been fully fitted out and armed.

Unlike the U.S. Navy’s longer-range nuclear carriers, both of China’s feature Soviet-design ski-jump bows, intended to provide sufficient take-off lift for fighter jets. They lack the powerful catapult launch technology that U.S. carriers have.

State media have quoted experts as saying China needs at least six carriers. The United States operates 10 and plans to build two more.

Most experts agree that developing such a force will be a decades-long task for China, but progress on a home-built carrier boosts prestige for Beijing, seen by many analysts as keen to eventually erode U.S. military prominence in the region.

Ren said he had no details to provide on whether China was building any other carriers.