The United States on Tuesday urged Iran to stop blocking online social media and advised its citizens to set up virtual private networks, or VPNs, to circumvent censorship.

Steve Goldstein, the State Department's under-secretary for public diplomacy, denounced Iran's attempts to restrict net access and urged Iranians to find a way to log in.

"People in Iran should be able to access these sites through VPN," he said, adding that the State Department's own Farsi language Facebook page has around 700,000 subscribers.

"The more available these sites are the better it is," he said, as street protests continued against what US President Donald Trump has branded Iran's "brutal and corrupt" regime.

Since the protests erupted, Iran has restricted some social media services like Instagram and Telegram that authorities fear will be used to spread news about the unrest.

Some other online services provided by US tech giants are unavailable in Iran because their parent firms are wary of falling afoul of economic sanctions targeting the regime.

But Goldstein said Washington believes that all Iranians should have access to non-government news and opinion, and urged them to listen to international broadcasters.

"We want to encourage the protesters to fight for what is right and to open up Iran," Goldstein told reporters.

"It's our strong desire that the Iranian government allow the protesters to dissent in peace."

Some observers warn Washington's public support for the right to oppose the Tehran government could tarnish the protest movement by making it appear like a foreign-backed plot.

But Goldstein insisted: "We have an obligation not to stand by. There's always that risk, but we're giving people vehicles to express their views where we can."