European SIM maker Gemalto said Wednesday it had suffered hacking attacks that were likely conducted by US and British intelligence agencies but denied any "massive theft" of encryption keys that could be used to spy on conversations.

Investigative website The Intercept last week said the US National Security Agency and Britain's GCHQ hacked into the firm in 2010 and 2011 and stole SIM encryption keys, with which they can reportedly monitor communications over mobiles without using a warrant or wiretap.

The website made the allegations on the theft of the keys -- which encrypt and decrypt data -- based on a document leaked by former NSA contractor Edward Snowden, and its report prompted some experts to decry a huge breach in mobile privacy.

"In 2010 and 2011, we detected two particularly sophisticated intrusions which could be related to the operation," Gemalto said in a statement.

"During the same period, we also detected several attempts to access the PCs of Gemalto employees who had regular contact with customers," it added.

"At the time we were unable to identify the perpetrators but we now think that they could be related to the NSA and GCHQ operation."

But the company denied that these attacks resulted in a large-scale theft of encryption keys.

"The attacks against Gemalto only breached its office networks and could not have resulted in a massive theft of SIM encryption keys," it said.

- Gemalto won't sue -

CEO Olivier Piou said the company would not file a complaint against the spy agencies as "the facts are difficult to prove from a legal standpoint and suing a state is long and costly."

The firm said the aim of the operation was to intercept the encryption keys as they were exchanged between mobile operators and suppliers such as Gemalto.

But "by 2010, Gemalto had already widely deployed a secure transfer system with its customers and only rare exceptions to this scheme could have led to theft."

Patrick Lacruche, group vice president in charge of security, told reporters that most customers used this system.

"But in some cases due to a specific emergency, tests or maintenance that needed doing, it's possible that some files did not go through secure channels," he said.

"But... this is very exceptional."

In its statement, the company said that in the case of a key theft, "the intelligence services would only be able to spy on communications on second generation 2G mobile networks."

"3G and 4G networks are not vulnerable to this type of attack," it added.

The NSA has come under intense scrutiny both at home and abroad after Snowden leaked documents from June 2013 about government surveillance programmes that sweep up data from Americans as well as foreigners.

The revelations led to a public outcry and strained relations with US allies.

Snowden, who fled the United States, has gained temporary asylum in Russia.

US President Barack Obama vowed to reform the country's surveillance programmes following the outcry, but the US Senate in November blocked a bid by lawmakers to curb NSA bulk data collection.

The USA Freedom Act surveillance reform bill that was blocked would have reined in the NSA and also replaced the agency's blanket authority with a far narrower one allowing it to obtain call records from phone companies but only in specific cases.