China has dismissed Google's allegation that Beijing is hampering access to its email service as "unacceptable". The internet provider said it believed government blocks were responsible for technical problems using Gmail from China.

The problems arose amid a tightening of internet controls that has made it increasingly difficult to use several popular virtual private networks. VPNs allow people to access material hosted overseas even if it is blocked by the Chinese government.

The new restrictions appear to be part of a security clampdown sparked by anonymous online calls for a "jasmine revolution" akin to the recent uprisings in the Middle East. Those messages were posted on an overseas website, but scores of Chinese activists and dissidents have been questioned, harassed and in some cases detained by the authorities for weeks.

A Google spokesman told the Guardian this week: "Relating to Google, there is no [technical] issue on our side. We have checked extensively. This is a government blockage carefully designed to look like the problem is with Gmail."

Users have reported frequent problems with basic tasks such as sending and searching emails or opening their address books.

But Jiang Yu, a spokeswoman for the foreign ministry, told a regular news conference: "This is an unacceptable accusation." She declined to comment further. The ministry of commerce and ministry of industry and information technology did not respond to faxed questions.

This month WiTopia, one of the most popular VPNs, asked users to report problems via email rather than its live support service because of an "extraordinary volume [of issues] from China shenanigans". The architect of the online censorship system had previously said it was "lagging behind" in a battle against VPNs and that further tightening was needed.

"It's grim. The reality is they can shut off all access if they want to," said Bill Bishop, a Beijing-based internet specialist.

He added: "You are heading into a two-internet world."

One Beijing-based industry source - who, like many, did not want to be identified in connection with the subject - said he hoped controls might ease in time. He pointed out that in the past sites such as Wikipedia had become available after having been blocked for years and suggested that blocks might be a temporary measure to encourage Chinese users to switch to local alternatives.

Many users are largely oblivious to the tightening of restrictions, preferring to use domestic email and social media services. But a growing number of activists and dissidents have embraced services such as Twitter, which is blocked and available only with the use of a VPN or other censorship-evasion technology.

Richard Buangan, spokesman for the US embassy in Beijing, said: "As part of our ongoing dialogue with China, we have emphasised to the Chinese government our view on the importance of an open internet. The ability to operate with confidence in cyberspace is critical in a modern society and economy.

"The United States believes that freedom of expression, including on the internet, is a universal right that should be available to all people, whether they are in the United States, China or any other nation."

Google angered the Chinese government when it announced last year that it was no longer willing to censor search results in the country and moved its Chinese search service to Hong Kong. It cited increased censorship and a cyberattack which it said appeared to have targeted human rights activists.

Separately, Google reported this month that it had seen "some highly targeted and apparently politically motivated attacks against our users", exploiting a vulnerability in Microsoft's Internet Explorer browser. It did not identify the subjects, but journalists in China reported suspicious messages from users such as "moli hua" - Chinese for "jasmine" - at the same time.

Greg Walton, of cyber intelligence company MetaLab Asia, who analysed those messages, said users were invited to click on links that led to malware hosted on a Hong Kong server. It was apparently designed to download Gmail cookies and email them to several addresses, enabling access to the targets' Gmail accounts. One piece of malware appears to have been designed to connect the target computer to a command and control server in Heilongjiang, northern China.

Chinese officials have repeatedly said their laws ban hacking and that the country is itself a victim of cyber-attacks.