By Jeremy Wagstaff

SINGAPORE, June 21 (Reuters) - Security researchers have many names for the hacking group that is one of the suspects for the cyberattack on the U.S. government's Office of Personnel Management: PinkPanther, KungFu Kittens, Group 72 and, most famously, Deep Panda.

But to Jared Myers and colleagues at cybersecurity company RSA, it is called Shell Crew, and Myers' team is one of the few who has watched it mid-assault - and eventually repulsed it.

Myers' account of a months-long battle with the group illustrates the challenges governments and companies face in defending against hackers that researchers believe are linked to the Chinese government - a charge Beijing denies.

"The Shell Crew is an extremely efficient and talented group," Myers said in an interview.

Shell Crew, or Deep Panda, are one of several hacking groups that Western cybersecurity companies have accused of hacking into U.S. and other countries' networks and stealing government, defence and industrial documents.

The attack on the OPM computers, revealed this month, compromised the data of 4 million current and former federal employees, raising U.S. suspicions that Chinese hackers were building huge databases that could be used to recruit spies.

China has denied any connection with such attacks and little is known about the identities of those involved in them.

But cybersecurity experts are starting to learn more about their methods.

Researchers have connected the OPM breach to an earlier attack on U.S. healthcare insurer Anthem Inc, which has been blamed on Deep Panda.

RSA's Myers says his team has no evidence that Shell Crew were behind the OPM attack, but believes Shell Crew and Deep Panda are the same group.

And they are no newcomers to cyber-espionage.

CrowdStrike, the cybersecurity company which gave Deep Panda its name due to its perceived Chinese links, traces its activities to 2011, when it launched attacks on defence, energy and chemical industries in the United States and Japan.

But few have caught them in the act.

SHELL CREW IN ACTION

In February 2014 a U.S. firm that designs and makes technology products called in RSA, a division of technology company EMC, to fix an unrelated problem. RSA realised there was a much bigger one at hand: hackers were inside the company's network, stealing sensitive data.

"In fact," Myers recalls telling the company, "you have a problem right now."

Myers' team could see hackers had been there for more than six months. But the attack went back further than that.

For months Shell Crew had probed the company's defences, using software code that makes use of known weaknesses in computer systems to try to unlock a door on its servers.