The messaging service WhatsApp has been disrupted in China as the government steps up security ahead of a Communist Party meeting next month.
Users have faced problems with the app for more than a week with services dropping in and out.
At times, it has been completely blocked and only accessible via virtual private networks (VPNs) which circumvent China's internet firewall.
WhatsApp is Facebook's only product allowed to operate in mainland China.
Facebook's main social media service and its Instagram image sharing app are not available on the mainland.
The BBC's China-based correspondents said the WhatsApp messaging service started going offline more than a week ago.
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A test of its services on Tuesday showed users in China could not send video messages or photographs to people outside China.
The disruption follows restrictions on WhatsApp video chats and photographs in July, which were later lifted.
The tightening of online censorship comes as China steps up security ahead of the Communist Party's national congress which is held every five years.
"The run-up period to a gathering is normally a time of greater restrictions of all kinds to assure that the critical Party Congress is held under ideal social conditions and is not disrupted", Robert Lawrence Kuhn, long-time advisor to China's leaders and multinational corporations told the BBC.
However, he said it is not yet clear whether the restrictions will be relaxed as has happened after previous party congresses, adding that many analysts do not believe they will be.
WhatsApp has declined to comment on the latest clampdown.
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