BEIJING, July 16 (Reuters) - TikTok owner ByteDance has been blocking new user and content creator registrations for its Chinese news aggregator Jinri Toutiao since September at the behest of regulators, according to people familiar with the matter. The halt on new registrations had not been previously reported. Some content creators have said on social media that they couldn"t register for new accounts on Toutiao. The reason for the blockage could not be immediately determined. Toutiao"s app can still be downloaded and existing users can use it. ByteDance declined to comment. The sources declined to be identified given the sensitivity of the matter. The Cyberspace Administration of China (CAC) and the Ministry of Industry and Information Technology, which oversee Toutiao"s operations, did not reply to requests for comment. China has been reining in its massive and once free-wheeling "platform economy" on concerns ranging from the appropriateness of content to anticompetitive behaviours and, increasingly, data security. Earlier this month, the CAC announced a probe into ride-hailing firm Didi Global (DIDI.N), citing cybersecurity and national security concerns, with a ban on new user registrations just days after it went public in a $4.4 billion New York share sale, rattling the industry and investors. The regulator followed up with the removal of Didi and its affiliated apps from Chinese app stores. Toutiao is ByteDance"s second-largest ad revenue source in China, after Douyin, Reuters has reported, accounting for around 20% of sales in the country last year, or about 36 billion yuan ($5.57 billion). ByteDance raked in $34.3 billion in total revenue last year. read more When trying to sign up to Toutiao, new users see a message: "System is currently under maintenance. Registration is temporarily unavailable." New content creators receive a similar message. Jinri Toutiao, which means "today"s headline" in Chinese, uses an artificial-intelligence-driven recommendation system to offer personalised news feeds. This is not the first time the news aggregator has been penalised by authorities. In 2018, Chinese regulators ordered app stores to remove Toutiao for three weeks, citing "vulgar content". ByteDance, which was valued at $300 billion in recent private trades, had planned earlier this year to list some of its businesses including TikTok"s Chinese version Douyin in Hong Kong, sources have said. In April ByteDance said it had no imminent plans to list itself. In 2018, ByteDance’s joke app Neihan Duanzi was permanently shut for hosting what the country"s broadcasting watchdog said was "vulgar" and "improper" content that had triggered "strong resentment among users". ($1 = 6.4653 Chinese yuan renminbi)