China's internet users have been cut off from accessing all foreign websites for around an hour in an unexplained incident that sparked speculation the country's censorship system was being tested or further tightened.

The "great firewall" already blocks many sites hosted from other countries, but users in Beijing, Shanghai and other parts of China reported that they could not reach any foreign sites whatsoever on Thursday morning – although it was not clear whether the problems were universal.

Meanwhile, users abroad and in Hong Kong – which is part of China but not subject to Beijing's net censorship – said they were unable to reach any sites on the Chinese mainland.

Some believed it was purely a technical failure, with several suggesting that Wednesday's massive earthquakes had hit an undersea cable, disrupting services. In 2007, a tremor hit a major cable and dramatically slowed access to overseas sites for months.

Xu Chuanchao, an executive at Sohu, one of the country's biggest internet portals, wrote on his microblog: "This malfunction is caused by the failure of China's backbone network and is under renovation."

But one company, Data Centre for China Internet, posted: "Latest news: most foreign websites can't be accessed. Analysis: for commonly known reasons, a large number of foreign URLs are blocked. It is possible that the great firewall is undergoing some readjustment, mistakenly adding many foreign websites to the blocking list. The details are unclear."

While many people reported problems with their virtual private network (VPN), lesser known VPNs seemed to connect without any problems. The problem also emerged and cleared up relatively quickly.

"It's possible they were short of capacity and that's why some people got through, but given that obscure VPNs were working I find that hard to believe," said David Wolf of Wolf Group Asia, which advises internet and other companies.

"My gut feeling is that it was a software upgrade. The fact it was updated in a couple of hours suggests someone woke up and realised."

China relies on cables running from three points, making it easy for access to be choked off intentionally or by accident.

"If you compare a map of the US or even the UK there are many, many more gateways," said Wolf.

Li Kaifu, the former boss of Microsoft and then Google in China, told his 12 million microblog followers: "All foreign websites are inaccessible!" adding emoticons to show disappointment, surprise and curiosity.

Another microblogger wrote: "The Chinese nation built a new Great Wall."

Internet users' suspicions were increased by the fact that the problems came amid a crackdown on "internet rumours".

Xinhua reported on Thursday that the authorities had removed more than 210,000 online posts and shut down 42 websites since March.

China's main internet providers said they were aware of the problems and were seeking an explanation. A spokesman at the ministry of industry and information technology said it was looking into the situation.