BEIJING (Reuters) - China will unveil its new generation J-20 stealth fighter jet at an air show next week, the air force said on Friday, the first public showing of a warplane China hopes will narrow the military gap with the United States.

The ability to project air power is key for China as it takes on a more assertive stance on territorial disputes with neighbors in the East China and South China seas.

The Pentagon has said the fifth generation stealth aircraft China is developing, the J-20 and the J-31, are necessary for China's air force to evolve from a mostly territorial force to one that can carry out both offensive and defensive operations.

The J-20 will give a flight demonstration at next week's China International Aviation and Aerospace Exhibition in the southern city of Zhuhai, the People's Liberation Army Air Force said in a statement on its official microblog.

Air force spokesman Shen Jinke said the J-20's production was proceeding according to plan and would assist in the air force's mission to "safeguard sovereignty and national security".

"This is the first public appearance of China's indigenously manufactured new generation stealth fighter jet," the air force said.

In June, it said the jet would enter service "in the near future".

The new Y-20 military transport aircraft will also give a flight demonstration at the seven-day air show, which is held every two years and opens on Tuesday.

China showed off the J-31 at the last Zhuhai air show in 2014, a show of muscle that coincided with a visit by U.S. President Barack Obama for an Asia-Pacific summit.

China hopes the J-31, still in development, will compete with the U.S.-made F-35 stealth aircraft in the international market, according to China military watchers and state media reports.

Some analysts have said photographs of the J-20 suggest China may be making faster-than-expected progress in developing a rival to Lockheed Martin's radar-evading F-22 Raptor.

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But others say China's defense manufacturers are still struggling to develop advanced engines that would allow its warplanes to match Western fighters in combat.

Beijing has been ramping up research into advanced new military equipment, including submarines, aircraft carriers and anti-satellite missiles, which has rattled nerves regionally and in the United States.

China says there is nothing unusual about its development of military technology, and that it is a reasonable course of action for every country that wants to defend its security.

(Reporting by Michael Martina; Editing by Robert Birsel)