Military analysts say a vessel spotted in a Chinese shipyard in rare, recent satellite images could be a new or upgraded class of nuclear-powered attack submarine.

It is not clear whether the submarine is a new model, an upgrade of an existing vessel or something else. But diplomats and analysts have been watching closely after a Pentagon report in November said the Chinese navy was likely in the next few years to build a new attack submarine with vertical launch tubes for cruise missiles.

Images obtained by Reuters from private satellite imagery provider Planet Labs and others circulating on social media show the submarine in a dry dock in Huludao Port in Liaoning province.

GRAPHIC: Chinese attack submarine under satellite scrutiny -

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Greenish covers shroud areas behind its superstructure and stern - parts of the vessel that could house missile launch tubes and a new, quieter propulsion system, analysts said. Such clear images of submarines in dry dock are seldom seen.

The submarine was out of the water between April 24 and May 4, and was later seen mostly submerged in the same place after the dry dock was flooded.

Singapore-based security scholar Collin Koh said there was a great deal of interest in the prospect of a new class of Chinese Type 093 "hunter-killer" submarine with vertical launch tubes for guided missiles. But he said the recent satellite images were too limited to definitively identify the vessel.

The images "are very interesting but it is still very hard to be sure yet whether we are looking at some kind of refit for testing or a whole new class of submarine," said Koh, of the S. Rajaratnam School of International Studies.

Koh said he and others were watching closely to see whether new Chinese submarines would shift to quieter pump-jets instead of conventional propellers for propulsion. Because the stern is shrouded, it is not possible to tell what sort of propulsion the ship in the image uses.

Vertical launch tubes would add considerable flexibility to China's "hunter killer" submarine fleet, arming the vessels with more guided missiles.

The Chinese Defence Ministry did not immediately respond to Reuters' requests for comment.

China's attack submarines are evolving to tackle a growing range of potential demands, from protecting ballistic missile submarines and the People's Liberation Army Navy's aircraft carrier battlegroups to tracking enemy ships.

Jeffrey Lewis, a professor in arms control at the Middlebury Institute of International Studies at Monterey, said the latest images raised more questions than answers as to whether they revealed a new class of vessel.

"It's plausible, but there are questions I'd want answered. It wasn't in dry dock very long and it is unclear how they may have reconfigured this submarine," he said. "Given the Pentagon report, there is certainly a lot of interest."