Iran has stepped up online censorship by upgrading the filtering system that enables the Islamic regime to block access to thousands of websites it deems inappropriate for Iranian users.

The move comes one month after the United States announced plans to launch new services facilitating internet access and mobile phone communications in countries with tight controls on freedom of speech, a decision that infuriated Tehran's regime and prompted harsh reactions from several Iranian officials.

The upgrade had at first appeared as a relaxation of the censorship machine. Iran's online community said on Monday that filtering was temporarily lifted for the entire country, giving users access to banned websites such as Twitter and Facebook. But hopes for an end to censorship were dashed when news agencies reported later in the day that the respite was due to the process of making the upgrade.

Despite the filtering, many Iranians access blocked addresses with help from proxy websites or virtual private network (VPN) services. The upgrade is aimed at stopping users bypassing censorship.

More than 5 million websites are filtered in Iran. Media organisations including the Guardian, BBC and CNN are blocked, though access to the New York Times website is allowed. On Google, the Farsi equivalents for words such as "condom", "sex", "lesbian" and "anti-filtering" are filtered out.

Iran is believed to be worried about the influence of the internet and especially social networking websites as pro-democracy activists across the Middle East use them to promote and publicise their movements.

Iran's semi-official Fars news agency quoted an unnamed official as saying: "The ministry of communications and information technology is strengthening the filtering system and recent disruptions were the result of this upgrade."

At the same time, Nasimonline.ir, an agency that publishes short Twitter-style bursts of news, said it had received information that "a new filtering system that targets Google and Yahoo search engines" had been installed and tested on Monday.

"I think that the new upgrade in the filtering system is a signal from Iran that the regime is prepared to stop any attempt by the US to challenge the country's online censorship," said an Iranian who spoke to the Guardian by phone from Tehran on condition of anonymity.

The New York Times reported last month that the Obama administration is leading a global effort to deploy what is known as "shadow internet" or "internet in a suitcase" by spending $2m (£1.25m) on secretive projects to create "independent cellphone networks inside foreign countries". The aim is to provide services that allow "wireless communication over a wide area with a link to the global internet" in support of dissidents in countries that have tightened their grip on freedom of speech.

In reaction to the US move, Iran's intelligence minister, Heidar Moslehi, was quoted by Fars as saying: "We had predicted these actions, such as the internet in suitcase, and we have planned proper ways to combat them."

In an interview with the semi-official Mehr news agency, Iran's minister for communications and information technology, Reza Taghipour, accused the US of "cyber terrorism" for its plans to launch "internet in suitcase".

In April, the Tehran government announced that it intended to launch "halal internet", a country-wide intranet and a parallel network that conforms to Islamic values with the ultimate goal of substituting for the global internet.

Iran's opposition believe that Iran is buying its filtering technology from China. In September 2009 Iran's powerful Revolutionary Guards, a body under the direct control of the supreme leader, Ayatollah Ali Khamenei, purchased 51% of the Telecommunications Company of Iran, which monitors internet filtering in the country.