The Chinese government has denied reports that it was involved in an attempt to steal user data from iCloud accounts within the country.
According to campaign website greatfire.org the service was subjected to a "man in the middle" (MITM) attack.
It took the form of a false website placed between the genuine log-in page and iCloud's servers.
Greatfire said it resembled other government-backed attacks but a spokesman denied any involvement.
"This is clearly a malicious attack on Apple in an effort to gain access to usernames and passwords and consequently all data stored on iCloud such as iMessages, photos, contacts, etc," said Greatfire in its report.
"If users ignored the security warning and clicked through to the Apple site and entered their username and password, this information has now been compromised by the Chinese authorities."
The attack coincided with the launch of Apple's iPhone 6 handset in the country last week.
Hua Chunying, a spokeswoman for China's foreign ministry, told journalists the government was "resolutely opposed" to hacking.
China Telecom, the country's state-owned internet provider, also said the accusation was "untrue and unfounded".
However, two security experts claim the allegations of government involvement are credible.
"Generally one has to be slightly careful about attributing these attacks - however, in this case, it does seem to have come from deep within in China's domestic network.
"The finger is definitely pointing in that direction," Prof Alan Woodward, a security researcher from the University of Surrey, told the BBC.
"It appears to have been a fairly classic MITM attack. [The Chinese government] has a reputation for hoovering up any data they can get but the big question is, what were they interested in?"
"All the evidence I've seen would support that this is a real attack," said Mikko Hypponen, chief research officer at F-Secure.
"The Chinese government is directly attacking Chinese users of Apple's products."