Sina.com.cn, the country's largest Internet portal, blamed "system pressure" for Wednesday morning's breakdown of their Twitter-like microblog service, contradicting some speculation that government agencies were behind the shutdown.

Sina's microblog, used by over 50 million people, became inaccessible Wednesday at about 10 am but service resumed 4 hours later, the Beijing News reported Wednesday.

The temporary breakdown came after microblogs run by sina.com.cn and other providers simultaneously reverted to testing modes in July, a move some observers described as a sign of a tight control on the widely used medium.

"Recently, the number of microblog users on Sina and the amount of messages posted daily have proliferated, it brought certain pressure to the system and triggered a breakdown," the state-ment said.

However, speculation over the cause of the breakdown didn't disappear given that China's first Twitter equivalent fanfou.com was blocked last year. Fanfou recently resumed service.

Guan Peng, deputy manager of china35.com, an Internet company that specializes in domain names, said he was "skeptical" about Sina's statement.

"It is almost impossible that the system of such a large company to face so much pressure as to collapse because usually there is a backup plan to handle the large Internet traffic," Guan told the Global Times.

He speculated that it was possible that some government agencies ordered Sina to suspend the operation in order to check for sensitive contents.

Cao Zenghui, deputy general manager of Sina's microblog business, declined to discuss the issue.

Tens of thousands of microblog users also expressed frustrations after the breakdown.

"I am going mad, when will this recover," wrote a user from Anhui below the statement posted on Sina's website.

"I did not realize how much I was obsessed with microblogs until this breakdown," wrote another user.

A few microblog websites appeared in China after Twitter gained popularity abroad. However, it was not until several major Chinese portals started operating this service late last year that the microblog became better known.

"We were stunned by the speed of microblog's development in China," Li Fang, the manager of Tencent's microblog business, said at a forum in Beijing last week.

However, both Li and Cao acknowledged microblogs are not officially licensed. "The authorities are still caught up in a wait-and-see attitude," Li said.