Iran blocked a ‘virtual embassy’ website launched by the US State Department hours after it was launched on Wednesday.
"In accordance with the cybercrime law, access to this website is not possible," read a notice to anyone inside Iran trying to visit iran.usembassy.gov.
The "Virtual Embassy Tehran" website - which makes clear it does not offer consular services - is an attempt by US Secretary of State Hillary Clinton to give Iranians a sympathetic view of the United States which is referred to as "the Great Satan" by Tehran's ruling elite.
The US has had no diplomatic relations with Iran since its embassy in Tehran was stormed in 1979 - the year of the Islamic Revolution - and its staff held hostage for 444 days. Since then, Washington has sought to isolate Iran, leading international sanctions against nuclear activities it believes are aimed at making the atom bomb - a charge Tehran denies.
The website was launched one week after British diplomats fled Iran after their embassy was stormed by radical youths, bringing Tehran's relations with Europe to a new low.
Tehran blames the West for seeking to overthrow its Islamic system and its control of the internet aims to stop Iranians accessing foreign sites that offer views that do not concur with its own tightly controlled media.
The semi-official Fars news agency commented on the blocking of the US website, saying: "a decisive reaction by Iranian authorities has neutralised another sly plot by the Americans".
Iranians will still be able to access the site by using a virtual private network (VPN) - software that can subvert the government filter and is essential for the millions of Facebook users in the Islamic state.