By Gerry Doyle

(Reuters) - Blurry images of two seemingly new Chinese military aircraft with stealthy characteristics appeared online on Friday, with defence experts saying they were clearly advanced designs, but there was not enough detail for definitive conclusions.

Both designs are tailless, meaning they do not have vertical stabilisers to help maintain control. Such aircraft are typically kept stable by computers that interpret the pilot's control inputs.

The larger of the two designs is roughly diamond-shaped, with three air intakes for its engines - two alongside the fuselage and one on top - an extremely unusual configuration. The smaller one has a more conventional layout, but no tail.

Both have the lack of 90-degree angles typical of stealth shaping, which is designed to reduce radar detection.

As China modernises its military, the designs "show the willingness of China's aviation industry to experiment and innovate", said Euan Graham, a senior analyst at the Australian Strategic Policy Institute.

"Whatever the merits or demerits, it appears to be a highly original design," he said. "They deserve kudos for that, and should shake off any lingering complacency that the U.S. and its allies always set the pace."

China's defence ministry did not immediately respond to a request for comment. The U.S. Department of Defense said it was "aware of the reports" but said it did not have additional comment beyond what was included in its annual report on the Chinese military this month.

SIXTH-GENERATION FIGHTER JETS

The United States is years into its Next Generation Air Dominance (NGAD) programme, which is developing a sixth-generation fighter aircraft, but it is not clear what form the effort will take under President-elect Donald Trump's incoming administration.

Europe and Asia are collaborating on a separate next-generation aircraft under the Global Combat Air Programme, and recently announced a joint British-Japanese company.

Reuters confirmed a video showing the larger of the two new Chinese aircraft flying over Chengdu by matching nearby buildings, signage, logos and trees to satellite imagery and file images. The date could not be verified independently.

The Chinese aircraft in the online images are not the first modern tailless designs. The Northrop Grumman B-2 and B-21 stealth bombers are both flying wings, and several uncrewed aircraft, such as the Lockheed Martin RQ-170 and China's CH-7, lack tails.

Neither Chinese jet has an official designation that has been made public. Although the designs are novel relative to the rest of China's fleet, it is not possible to tell how stealthy they are, how manoeuvrable or fast they are, or what sort of "under the hood" avionics they carry - the sorts of detail needed to determine whether they are truly "next-generation" designs, five defence experts said.

Peter Layton, a defence and aviation expert at the Griffith Asia Institute in Australia, noted it was difficult in general to apply labels such as fifth-generation - which includes current stealth jets such as the F-22 and F-35 - to Chinese aircraft, which often include unique design features that never appear again.

China's J-20 and J-35 also have stealthy characteristics but their capabilities are not publicly known and only the J-20 is in service.

The United States has been pouring resources into advanced missiles and other systems to deter China in the Indo-Pacific region. It recently tested a missile interception system on the strategically crucial island of Guam using an advanced radar.

China's daytime flights of the new designs over cities where they could be easily seen was "curious", said Kelly Grieco, a senior fellow at the U.S.-based Stimson Center.

She said it might be easier for China to match pace with the United States on aircraft rather than uncrewed vehicles and missiles.

"As the Pentagon is actively debating the future of NGAD ... it is hard not to wonder if this is Beijing's attempt to influence that debate," she added.

Separately on Friday, Chinese state media outlet Xinhua reported the People's Liberation Army Navy had launched its newest amphibious assault ship.

(Reporting by Gerry Doyle in Singapore; Additional reporting by Ryan Woo in Beijing; Editing by Jamie Freed)