"The spacecraft, which blasted off on Wednesday morning with two astronauts aboard, has slightly deviated from its designed orbit and (is) moving a little closer to the Earth due to gravity," the Xinhua news agency said.

As a result, the spacecraft will undergo its first orbit maintenance operation in the early hours of Friday in order to restore the vessel to its original trajectory, Xinhua said.

Xinhua cited unnamed experts saying the maintenance will be a "normal technical operation" and similar tasks could be carried out in the future in accordance with the spacecraft's orbiting situation.

A US-based China space expert told AFP that while all orbits require some level of maintenance, unless the spacecraft goes out to 22,000 mileskilometers) from Earth, the deviation was still unusual and "disturbing."

"Especially for a manned craft ... you're going to know what orbital path you're going to put it in and you're going to expect it to stay there," said Dean Cheng, senior Asia analyst with the Virginia-based nonprofit technical analysis thinktank Center for Naval Analyses or CNA Corporation.

"In and of itself, it's not a big deal. What they haven't said is how after such extensive planning, why are they making changes? The question is what went wrong?"

Cheng said he believed China could put the spacecraft back on its correct orbit.

"Can they do it? Yes. It's not a particularly difficult, it's not a particularly dangerous maneuver," Cheng said. "The pilots themselves can do it or they can do it from ground control."

He likened the maneuver to the US space shuttle moving from one orbit into different one to make a docking maneuver, such as to carry out repairs to the Hubble Space Telescope.

"This is the equivalent kind of maneuver in the sense that you are moving from where you are to where you want to be. In the case of the shuttle, those are planned maneuvers."

"What I'm hearing is the Chinese have planned maneuvers but they weren't planning on doing it now. And if that's the case then this becomes a very serious issue because the question becomes why?" Cheng said.

Xinhua, the government's mouthpiece on the space flight, did not provide further information.

Shenzhou VI is expected to travel in space for up to five days, exceeding the time spent in space during China's first manned flight in 2003, when astronaut Yang Liwei aboard Shenzhou V spacecraft stayed in space for only 21 hours, orbiting the earth 14 times.

All rights reserved. copyright 2018 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.