China has the largest armed forces in the world but when it comes to military might, the country's latest effort to close the gap with the United States faces an "embarrassing" problem.

After a long and secretive campaign, Beijing recently announced its new generation J-20 stealth fighter warplane is combat ready. The fleet of jets (Beijing won't say how many there are) has been armed and officially commissioned into China's air force, the Communist Party said in a statement over the weekend AEST.

"The stealth jets will improve the air force's comprehensive fighting ability and enable it to better safeguard China's sovereignty, security and territorial integrity," Shen Jinke, spokesperson for the People's Liberation Army (PLA) Air Force, was quoted saying by local media.

However according to sources close to the military project, that's not the entire story.

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Shortly after the announcement a report in the South China Morning Post, a major Hong Kong newspaper, claimed sources said there was an "embarrassing" engine flaw that will hinder the warplane if it does enter any combat scenario.

Due to problems in development, the warplanes aren't equipped with the WS-15 engines they were built to fly with.

"The WS-15 engine designed for the J-20 exploded during a ground running test in 2015," a source told the paper.

The incident prompted the PLA to install a different engine, the WS-10B, which is a modified version of the WS-10 Taihang engine, designed for the country's fourth-generation J-10 and J-11 fighters from 1998 and 2002.

As a result the manoeuvrability and fuel efficiency as well as its stealth capacity at supersonic speeds is less than ideal. And far from competitive with its US counterparts.

Without the new WS-15 engines, the J-20 can't supercruise, or fly above the speed of sound without igniting its afterburners, meaning at top speeds the stealth fighter isn't so stealthy.

China's J-20 stealth fighter is now combat ready for those who want to
China's J-20 stealth fighter is now combat ready for those who want to "provoke" China.

"It's so embarrassing to change engines for such an important aircraft project several times," the source reportedly said.

"It is the longstanding core problem among home-grown aircraft."

Chinese President Xi Jinping is overseeing a sweeping modernisation of the country's armed forces including anti-satellite missiles and advanced submarines, seeking to project power far from its shores.

The J-20 will further raise the air force's combat abilities and help the air force better carry out its "sacred mission" to defend the country's sovereignty, security and territorial integrity, the air force said.

State media reported late last year that the J-20 aircraft had entered service. The aircraft was shown in public for the first time in late 2016 at the Zhuhai airshow.

But questions also remain about whether the new Chinese fighter can match the radar-evading properties of the Lockheed Martin F-22 Raptor air-to-air combat jet, or the latest strike jet in the US arsenal, Lockheed's F-35.

The F-22, developed for the US Air Force, is the J-20's closest lookalike.

Nonetheless, China continues to be exceedingly keen to project its power and improved capabilities in air combat.

In a post on the PLA's English-language website, Chinese military expert Song Zongping said the J-20 will "engage with rivals in the future who dare to provoke China in the air."

At the Zhuhai airshow in 2014, China showed off another stealth fighter it is developing: the J-31.

The country hopes the J-31, still in development, will compete with the US-made F-35 stealth aircraft in the international market, according to state media reports.

With AAP