Beijing was unusually open in revealing its satellite capabilities when it released photographs of possible debris from a missing airplane, despite taking four days to make the images public, analysts said Thursday.

China's State Administration for Science, Technology and Industry for National Defence (SASTIND) published three pictures late Wednesday of what it said were suspected large floating objects in the South China Sea.

The images were taken on Sunday, it said, raising questions as to why it took several days for them to emerge, and whether -- and if so, when -- they had been passed to the Malaysian authorities co-ordinating the search.

China's space programme is military-run and normally shrouded in secrecy.

Malaysian and Vietnamese flights in the area of the photographs failed to spot anything, officials said.

But Morris Jones, an independent space analyst based in Australia, said Beijing's disclosure of the pictures was surprisingly open.

"Satellite imagery is a strategic tool that has military applications, and nations are usually very cautious in revealing how much these satellites can do and how much they can see," he told AFP.

"I am surprised that the Chinese have openly released this image because we don't normally see images of this quality."

According to SASTIND, the objects were approximately 13 by 18 metres, 14 by 19 metres and 24 by 22 metres in size.

The actual images "were probably of a much higher quality than the images they released to the media", Jones said. Authorities released them "to provide enough information to show something, but the image is degraded to hide the true capabilities of the actual satellite."

While China's abilities in space remain decades behind the US, it has made rapid technological leaps and aims to launch an independent space station by 2020, eventually sending a man to the moon.

It has deployed 10 satellites in the search for MH370, Premier Li Keqiang said Thursday.

The image data was obtained by the China Centre for Resources Satellite Data and Application (CRESDA), an agency jointly overseen by SASTIND and China's National Development and Reform Commission (NDRC).

Among the agency's goals is the creation of a national-level satellite data centre to provide high-resolution images for national defence as well as civilian purposes such as agriculture and disaster planning.

It started surveying the area after hearing of the disappearance on Saturday, it said in a statement, and by Tuesday morning, the satellites had gathered data covering a total of 120,000 square kilometres -- an area almost the size of Greece.

"The quality of the data images is rather good," it said.

An official with CRESDA told AFP on Thursday that the agency was still analysing the images but that no conclusions had been reached yet.

Jones said he did not find "anything suspicious at all" in the delay in releasing the pictures.

"There's a lot of time you go through between the time you get the images and the time you release them," he said.

"The satellite has to get into position, take the photo, then relay it to a ground station, then it has to be analysed. And keep in mind, the analysts on the ground are probably looking through a tremendous amount of data. So I would say there's absolutely nothing abnormal."

One of Beijing's milestones in satellite technology came in late 2012, when it announced that it had launched a domestic satellite navigation network to rival the US GPS global positioning system.

The Beidou system -- named after the Chinese term for the plough or Big Dipper constellation -- currently has 16 satellites, a number expected to grow to 30 by 2020, when the system is slated to reach global coverage.

China is investing more than 400 billion yuan ($65 billion) in the system, which is expected to "benefit both the military and civilians", according to Chinese state media.