Reports say documents were stolen from contractors including Elisra Group, Israel Aerospace Industries and Rafael Advanced Defense Systems.
A security expert has claimed China-based hackers have stolen hundreds of documents related to Israel's Iron Dome missile shield from three contractors involved in the project.
According to the Krebs on Security blog, run by former Washington Post security reporter Brian Krebs, the Chinese hackers have stolen the documents by hacking into the networks of Elisra Group, Israel Aerospace Industries and Rafael Advanced Defense Systems.
The blogger claimed that state-sponsored Comment Crew hacking group, believed to be based in China, has hacked into the networks between October 10, 2011 and August 13. 2012, citing US-based threat intelligence firm Cyber Engineering Services.
The five Chinese military officials indicted by the US in May over alleged hacking into energy firms are also members of the same unit, according to Krebs On Security.
According to the Israel Defence Forces (IDF), several rockets launched from Gaza, mainly during the ongoing military conflicts have been intercepted by the system, which is believed to be one of the most efficient missile-defence technologies in the world.
Cyber Engineering Services is said to have told Krebs that Israel Aerospace Industries was hit with an email phishing attack, which exfiltrated about 700 files of 762MB, including emails, PDFs, scripts and spreadsheets.
CyberESI chief executive Joseph Drissel told Krebs that documents stolen include specifications for the Arrow III system and other ballistic missile defences.
However, Rafael Advanced Defense Systems has denied such a data hack.
An official told Reuters: "Rafael does not recall such an incident. Rafael's databases, including its air defense databases, are extremely well protected."
IAI senior VP for Communications Eliana Fishler also denied any leakage of sensitive information.
"The information reported regarding the leakage of sensitive information is incorrect,"Fishler said.
"IAI's cyber security systems operate in accordance with the most rigorous requirements and also in this case they were proven to be effective."