BOSTON/BEIJING (Reuters) - Apple Inc's iCloud storage and backup service in China was attacked by hackers trying to steal user credentials, a Chinese web monitoring group said, adding that it believes the country's government is behind the campaign. Using what is called a "man-in-the-middle" (MITM) attack, the hackers interposed their own website between users and Apple's iCloud server, intercepting data and potentially gaining access to passwords, iMessages, photos and contacts, Greatfire.org wrote in its blog post. Greatfire.org, a group that conducts research on Chinese Internet censorship, alleged government involvement in the attack, saying it resembled previous attacks on Google Inc , Yahoo Inc and Microsoft Corp's Hotmail. Asked about the attack, Hua Chunying, a spokeswoman for China's Foreign Ministry, told a daily news briefing that Beijing was "resolutely opposed" to hacking.