China has largely blocked the WhatsApp messaging app, the latest move by Beijing to step up surveillance before a big Communist Party gathering next month.

The disabling in mainland China of the Facebook-owned app is a setback for the social media giant, whose chief executive, Mark Zuckerberg, has been pushing to re-enter the Chinese market, and has been studying the Chinese language intensively. WhatsApp was the last Facebook product to still be available in mainland China; the company's main social media service has been blocked in China since 2009, where its Instagram image-sharing app is also unavailable.

In mid-July, Chinese censors began blocking video chats and the sending of photos and other files using WhatsApp, and they stopped many voice chats as well. But most text messages on the app continued to go through normally. The restrictions on video, audio chats and file sharing were at least temporarily lifted in the following weeks.

WhatsApp now appears to have been broadly disrupted in China, even for text messages, suggesting that China's censors may have developed specialised software to interfere with such messages, which rely on an encryption technology that is used by few services other than WhatsApp.

"This is not the typical technical method in which the Chinese government censors something," said Nadim Kobeissi, an applied cryptographer at Paris research start-up Symbolic Software. He added that his company's automated monitors had begun detecting disruptions of WhatsApp in China last Wednesday, and that by Monday the blocking efforts were comprehensive.

Facebook declined to comment, following past practice when asked about WhatsApp's difficulties in China.

Chinese authorities have a history of mostly, but not entirely, blocking internet services, as well as slowing them down so much that they become useless. The censorship has prompted many in China to switch to communications methods that function smoothly and quickly but that are easily monitored by Chinese authorities, like the WeChat app of the Chinese internet company Tencent, which is based in Shenzhen.

The disruption of WhatsApp comes as Beijing prepares for the Communist Party's congress, which starts on October 18.

Held once every five years, the congress chooses the party's leadership, which in turn runs the country. Next month's meeting is expected to reconfirm President Xi Jinping's nearly absolute grip on power, but considerable uncertainty remains over who will join him on the Standing Committee of the Politburo, the party's highest-ranking group.

Over the past several years, China has not only stepped up censorship but has also closed numerous churches and jailed large numbers of human rights activists, lawyers and advocates for ethnic minorities.

The shutdown of WhatsApp prompted considerable dismay on Chinese social media.

"Losing contact with my clients, forced back to the age of telephone and email for work now," one user complained on Weibo, a Twitter-like microblogging site.

"Even WhatsApp is blocked now? I'm going to be out of business soon," another Chinese social media user said on Weibo.

In China, even the use of email is fading as residents embrace the convenience of WeChat. The messaging service, which has 963 million active users, bears some similarities with WhatsApp but has a wider array of features and one crucial difference: very close ties to the government. This month, WeChat sent a notice to users reminding them that it complied with official requests for information.

When China joined the World Trade Organization in 2001, it agreed to open online data services and other enhanced telecommunications services to international competition. But it obtained the assent of other WTO members to retain restrictions on the media. Technology multinationals, heavily dependent on the Chinese market, have been reluctant to accuse Beijing of falling short of its WTO commitments.

The Office of the US Trade Representative has opened a formal investigation into whether China is violating the intellectual property of US companies, but it has released few details. The trade office has not said whether the inquiry will include the blocking of products that rely on US intellectual property, or whether it will focus more narrowly on cases in which China has allegedly purloined or otherwise copied it.

WhatsApp has a strong reputation among cryptographers for security, which may have been what drew the attention of Chinese censors. The app provides so-called end-to-end encryption, which effectively means that even Facebook does not know what is being said in the text, voice and video conversations passing through its servers.

Residents of mainland China can still use services like WhatsApp if they first connect to virtual private networks that provide them with communications channels to servers outside the Chinese mainland. But the government has been cracking down on VPNs in recent months as well, and even when those networks appear to be working, they sometimes do not allow access to services that the government is particularly targeting.

New York Times