Direct access to Facebook and Twitter was briefly restored in Iran on Monday evening.
The social media sites have been blocked since 2009 after they were used to organise protests.
On Monday the block was briefly lifted for some Iranians - but the authorities blamed "technical problems", stressing the official policy had not changed.
The hitch comes as US trade sanctions force the closure of an Iranian opposition leader's website.
Late on Monday, Western journalists working in Iran reported that they had suddenly gained direct access to Twitter and Facebook.
Thomas Erdbrink, Tehran bureau chief for the New York Times, sent several tweets via his mobile phone without the need to go via a proxy that circumvents official blocks.
Most Iranians who want to use social media have typically used proxies or other special software to get around the government imposed firewall.
Iranians reacted with cautious optimism when they realised Facebook and Twitter were freely accessible.
"If it is true, I think they have to register today in calendar as a day of Free Filtering," user Abbas Farokhi told BBC Persian.
Another, Benyamin HM, said: "Do not get over excited, it has happened because of some technical problems. It will be filtered again and we have to use proxy."
Initially it was thought the block was being lifted for everyone in Iran, signalling the start of a more tolerant attitude towards social media by the government.
In recent weeks some Iranian government officials and ministers have signed up for accounts on Facebook and Twitter.
This led to confusion earlier this month over whether President Hassan Rouhani had tweeted "Happy Rosh Hashana" to Iran's Jewish community. A message came from a Twitter account thought to belong to Mr Rouhani, but officials later said he did not have any such account.
Early on Tuesday Reuters reported that official policy had not changed and "technical faults" had mistakenly led to the restrictions being lifted.
It quoted Abdolsamad Khoramabadi, who heads the committee that oversees Iran's net filters, as saying it was looking into the hitch.
"We are investigating to see which of these companies has done this," he said.
At the same time, the website of Iranian opposition politician Mehdi Karoubi has been shut down by the Just Host service at the request of the US Treasury Department.
Sanctions restrict what services US companies can supply to Iranians and this extends to hosting websites that use the country's .ir domain suffix.
An aide for Mr Karoubi said the action by the US was "frustrating".