VIENNA, July 29 (Reuters) - Three Israeli defence contractors behind the Iron Dome missile shield and related systems were themselves robbed of hundreds of documents by hackers linked to the Chinese government starting in 2011, according to an independent U.S. security researcher.

Krebs on Security, a blog operated by former Washington Post security reporter Brian Krebs, reported on Tuesday that Iron Dome’s manufacturers were infiltrated by the state-sponsored Comment Crew hacking group, believed to operate out of China.

The targets of these online attacks were top military contractors Elisra Group, Israel Aerospace Industries , and Rafael Advanced Defense Systems, who were responsible for constructing the system which now partially insulates Israel from rocket barrages fired from the Gaza Strip.

Israeli and U.S. officials have said Iron Dome systems are responsible for shooting down more than 90 percent of the rockets they have engaged, while ignoring missiles on a trajectory to fall wide. That accounts for about a fifth of the rockets Israel has said militants have fired into the country during the latest crisis.

Two of the companies named declined to comment on the story or confirm whether the incidents, said to have occurred repeatedly throughout 2011 and 2012, indeed took place.

An official at the third company, Rafael Advanced Defense Systems, who declined to be identified by name, said of the report: “Rafael does not recall such an incident. Rafael’s databases, including its air defense databases, are extremely well protected.”

A former senior Israeli military official said assertions these key defence contractors were hacked would fit with a pattern of military and industrial espionage around the globe.

“First, this is old stuff from about two years ago”, said Uzi Rubin, a former head of missile defence at Israel’s Defence Ministry and now head of the Rubicon consultancy firm. “The Chinese have been doing that to all defence contractors in the West, so if this really happened we are not alone.”

Krebs said details of the attacks were provided by Columbia, Md.-based network intelligence firm Cyber Engineering Services Inc. (CyberESI) which traced the intrusions and identified more than 700 stolen emails, documents and manuals pertaining to development of the Iron Dome project and other missile projects.

Columbia, Md.-based CyberESI and its chief executive, Joseph Drissel, did not immediately respond to interview requests.

Drissel was quoted by Krebs as saying that stolen materials included specifications for the Arrow III system and other ballistic missile defences, much of the technology for which was developed by Boeing and other contractors for use in U.S. weapon systems.

Rubin speculated that if the purpose of the Comment Crew hacking group was to steal the plans for the missile system, the likelihood was that it was for China to obtain technology on the cheap rather than in order to resell it to other nations.

“If the Chinese really did it, maybe we shall see a Chinese Iron Dome in the future,” he told Reuters. “It is said that imitation is the sincerest form of a compliment.”

Chinese officials were not immediately available for comment.

Allegations of hacking and other espionage have also strained ties between China and the United States, with Beijing denying last year that it had set up a special military unit to conduct such activity.

The United States recently charged five Chinese military officers, accusing them of hacking into American companies to steal trade secrets. China retaliated by shutting down a bilateral working group on cyber security. (Reporting by Dan Williams in Jerusalem and Eric Auchard in Vienna, Editing by Angus MacSwan)