BEIJING—The first clear pictures of what appears to be a Chinese stealth fighter prototype have been published online, highlighting China's military buildup just days before U.S. Defense Secretary Robert Gates heads to Beijing to try to repair defense ties.

The photographs, published on several unofficial Chinese and foreign defense-related websites, appear to show a J-20 prototype making a high-speed taxi test—usually one of the last steps before an aircraft makes its first flight—according to experts on aviation and China's military.

The exact origin of the photographs is unclear, although they appear to have been taken by Chinese enthusiasts from the grounds of or around the Chengdu Aircraft Design Institute in western China, where the J-20 is in development. A few experts have suggested that the pictured aircraft is a mock-up, rather than a functioning prototype of a stealth fighter—so-called because it is designed to evade detection by radar and infrared sensors.

But many more experts say they believe the pictures and the aircraft are authentic, giving the strongest indication yet that Beijing is making faster-than-expected progress in developing a rival to the U.S. F-22—the world's only fully operational stealth fighter.

China's defense ministry and air force couldn't be reached to comment on the latest photos. Even without official confirmation, however, the photographs are likely to bolster concerns among U.S. officials and politicians about China's military modernization, which also includes the imminent deployment of its first aircraft carrier and "carrier-killer" antiship ballistic missiles.

Such weapons systems would significantly enhance China's ability to hinder U.S. intervention in a conflict over Taiwan, and challenge U.S. naval supremacy in the Asia-Pacific region.

Gen. He Weirong, deputy head of China's Air Force, announced in 2009 that China's first stealth fighters were about to undergo test flights and would be deployed in "eight or 10 years." But there was no clear physical evidence of their existence until the latest photographs emerged.

Chinese authorities who monitor Internet traffic in the country appear not to have tried to block the J-20 pictures.

"The photos I've seen look genuine," said Gareth Jennings, aviation desk editor at Jane's Defence Weekly.

"It's pretty far down the line," he said. "The fact that its nose wheel is off the ground in one picture suggest this was a high-speed taxi test—that usually means a test flight very soon afterwards. All the talk we've heard is that this could happen some time in the next few weeks."

U.S. officials played down Chinese advances on the plane, which American intelligence agencies believe will likely be operational around 2018. "We are aware that the Chinese have recently been conducting taxi tests and there are photos of it," said Pentagon spokesman Col. David Lapan. "We know they are working on a fifth-generation fighter but progress appears to be uneven."

Col. Lapan said it appears the Chinese are still seeking engines for a fourth-generation fighter from Russia, an indication that they are "still encountering problems" with development work toward the fifth-generation aircraft, the J-20.

But the 2018 estimate suggests U.S. officials believe China's development of the fifth-generation fighter has accelerated. In 2009, Mr. Gates predicted that China wouldn't deploy a fifth-generation fighter until 2020. U.S. officials said the latest disclosures wouldn't affect any U.S. aircraft-development programs.

China has made rapid progress in developing a capability to produce advanced weapons, also including unmanned aerial vehicles, after decades of importing and reverse engineering Russian arms. The photographs throw a fresh spotlight on the sensitive issue of China's military modernization just as Washington and Beijing try to improve relations following a series of public disputes in 2010.

Defense Secretary Gates is due to begin a long-delayed visit to Beijing on Sunday—almost exactly a year after China suspended military ties in protest over U.S. arms sales to Taiwan.

China's President Hu Jintao is then due to begin a state visit to the U.S. on Jan. 19. President Barack Obama joined in preparatory talks at the White House on Tuesday between his national security adviser, Tom Donilon, and Chinese Foreign Minister Yang Jiechi. During the meeting, Mr. Obama said he was committed to building a bilateral relationship that is "cooperative in nature," the White House said.

The two countries clashed last year over issues including the value of the Chinese currency, China's territorial claims in the South China Sea and vocal U.S. support for a jailed Chinese dissident who won the Nobel Peace Prize.

The U.S. was also frustrated by China's refusal to condemn two North Korean attacks on South Korea, while Beijing was angered by a U.S. decision to respond to the second attack, the shelling of a South Korean island in November, by sending an aircraft carrier to take part in joint naval exercises with Seoul near China's coast.

The U.S. and its Asian allies have also been alarmed by China's naval maneuvers and more forceful stance on territorial issues, while China's military strategists have accused the U.S. of trying to "contain" China—most recently by sending two more aircraft carriers to the region in December.

"The U.S. wants to retain its global hegemony and also preserve its regional interests. It is not comfortable with China's military rise," Senior Col. Han Xudong, a professor at China's National Defense University, was quoted as saying in the Global Times newspaper Tuesday.

Experts who said they thought the photographs were authentic included Andrei Chang of the Canadian-based Kanwa Asian Defence Monthly, and Richard Fisher, an expert on the Chinese military at the International Strategy and Assessment Center in Washington.

Several experts said the prototype's body appeared to borrow from the F-22 and other U.S. stealth aircraft, but they couldn't tell from the photographs how advanced it was in terms of avionics, composite materials or other key aspects of stealth technology.

They said that China was probably several years behind Russia, whose first stealth fighter, the Sukhoi T-50, made its first flight in January 2010, but that Beijing was catching up faster than expected.

The U.S. cut funding for the F-22 in 2009 in favor of the F-35, a smaller, cheaper stealth fighter that made its first test flight in 2006 and is expected to be fully deployed by around 2014. The F-22 has mainly been used for exercises and operations around U.S. airspace, but some have been deployed to Guam and Okinawa to help maintain the U.S. security umbrella in the Asia-Pacific region.

The Chinese prototype looks like it has "the potential to be a competitor with the F-22 and to be decisively superior to the F-35," said Mr. Fisher. The J-20 has two engines, like the F-22, and is about the same size, while the F-35 is smaller and has only one engine.

China's stealth-fighter program has implications also for Japan, which is considering buying F-35s, and for India, which last month firmed up a deal with Russia to jointly develop and manufacture a stealth fighter.