A photograph of what is reported to be a new Chinese stealth fighter and "carrier-killer" missile has prompted concerns that a tilt in the balance of military power in the western Pacific towards China may come sooner than expected.

The emergence of the hi-tech weaponry - which would make it more difficult for the US navy and air force to project power close to Taiwan and elsewhere on China's coastline - comes at a politically sensitive time. Later this month, President Barack Obama and his Chinese counterpart, Hu Jintao, will hold a summit in Washington aimed at patching up their differences after a niggling year in bilateral relations.

The photograph, of what appears to be a prototype J-20 jet undergoing runway tests, has been circulating on the internet since last week, fuelling speculation that China's fifth-generation fighter may fly ahead of forecast.

The defence ministry has yet to comment on the image, which seems to have been shot from long-distance near the Chengdu aircraft design institute. The photographer is also unknown, which has added to the mystery about its origins and authenticity as well as the motive of the distributor.

But defence analysts believe this is the first glimpse of the twin-engined, chiselled-nosed plane that mixes Russian engine technology with a fuselage design similar to that of the US air force's F-22 "stealth" fighter, which can avoid detection by radar.

If confirmed, it would be an impressive step forward for the Chinese air force, which until now has largely depended on foreign-made or designed planes. "I'd say these are, indeed, genuine photos of a prototype that will make its maiden flight very soon," said Peter Felstead, the editor of Jane's Defence Weekly.

The J20 is likely to be many years from deployment, but the US defence secretary, Robert Gates - who visits Beijing next week - may have to revise an earlier prediction that China will not have a fifth generation aircraft by 2020.

It is not the only challenge to US superiority in the region. China has refurbished a Ukranian aircraft carrier and wants to build its own by 2020.

A more immediate threat is posed by China's adaptation of an intermediate-range ballistic missile - the DF-21D - to target US aircraft carriers. This project is also further advanced than previously believed.

Admiral Robert Willard, the US navy's commander in the Pacific, warned last month that the weapon - nicknamed the "carrier killer' - had reached "initial operational capability". Faced by this threat US battle groups are likely to take a more withdrawn position if there is a standoff over Taiwan than they did in 1996, when the USS Nimitz sailed through the strait.

"The main implication of China deploying this system is that it would certainly make the US navy pause before deciding to project naval power into the South China Sea region during a time of tension," said Felstead.

But China's ambassador to the UK, Liu Xiaoming, said today that his country had no ambitions to rival US military power in the western Pacific region.

"We do not see ourselves as rivals to the US. We believe the US and China can work together in the region," Liu said, arguing there was a double standard in the west towards Chinese defence spending.

"When China carries out an exercise on its own territory there is a lot of attention, but when the United States comes all the way across the Pacific for exercises with its allies, no one speaks about it in the same way. There is a cold war mentality still. If you develop your defence capability, they [the Americans] are annoyed. But our defence construction is purely for self-defence. China's defence expenditure is still the lowest among the five permament members of the [UN] security council."

China's military advances worry many in the east Asian region who have benefited for decades from the US-policed status quo. In its latest defence white paper, Japan noted that China's military spending had nearly quadrupled over the past decade, while its own shrank by 4% due to a stagnant economy.

Officials in Tokyo have also expressed alarm at the increasingly confrontational approach of Chinese vessels in disputed fisheries. In Washington, rightwing thinktanks and commentators want Obama and Gates to apply diplomatic pressure on China to join the intermediate nuclear forces (INF) treaty and halt its missile buildup.

The photographs of the J20 jet are also likely to prompt calls for accelerated production of F22s - the US's most advanced stealth fighter - to ensure air superiority.

The US remains the most potent military force in the western Pacific with 60,000 troops, a military airbase in Okinawa and one forward-deployed carrier fleet.

The US also outspends China on defence by a ratio of six to one, according to the Stockholm International Peace Research Institute. Even so, while China's economy grows rapidly and the US remains sluggish, fears of a shift in the balance of power are likely to grow. It will not happen overnight and worldwide, but China appears to be steadily pushing the US back from its shores in a strategy know as "area denial". The government has not confirmed this approach. Chinese nationalists want their country to be more assertive, but they say the priority is to improve defence of an increasingly wealthy coastal region.

The "area denial" strategy can be seen as China trying to manage its own market and routes to main trading partners such as South Korea and Japan.

"We don't need the US to be the policeman in the west Pacific area," said Song Xiaojun, a former naval officer who now edits military magazines.

"China's priority is to develop its near sea defence, because our economy is concentrated on the coast. But we have to reconsider the concept of 'near sea' to fit a modern age in which military threats can come from far away. China must improve its defences, but that does not mean we are a threat. Only arms merchants would say that to persuade the US to raise military spending. The US is far ahead," he said.

• This article was amended on 14 January 2011. In the original, the next US generation of stealth fighter was given as the F35. This has been corrected.