The United States on Wednesday tightened sanctions on Iran to further choke off its oil income, saying it was necessary to increase pressure on Tehran over its suspected nuclear weapons program.

It also set sanctions against Iranian media organizations and Tehran's Cyber Police for what it called human rights abuses for censorship.

The US Treasury said that countries continuing to buy Iranian oil would have to retain their payment for the oil, and only allow it to be used for Iranian purchases of goods from them.

That would tighten Tehran's ability to freely use the money it gets from oil exports, which have already been sharply constricted by international sanctions on the country.

"So long as Iran continues to fail to address the concerns of the international community about its nuclear program, the US will impose tighter sanctions and intensify the economic pressure against the Iranian regime," said David Cohen, Treasury under secretary for terrorism and financial intelligence.

The move on oil revenues came six months after the US said it would deny access to the US financial system to countries buying Iranian oil, with certain countries given exceptions to wind down their trade.

These new rules narrow the exceptions, the Treasury said, and require any country still buying Iranian oil to credit the payment to a domestic account.

"In addition to effectively 'locking up' Iranian oil revenue overseas, this provision sharply restricts Iran's use of this revenue for bilateral trade and severely limits Iran's ability to move funds across jurisdictions," the Treasury said in a statement.

"We are working to make the choices for the Iranian leadership as stark as possible," said a senior US administration official, speaking on background.

"Iran can either meet its international obligations... or it will face increased pressure and financial isolation."

Iran blasted the new move, as it fends off global pressure over its nuclear program, which it says is for peaceful purposes only.

"This is the latest ring in the series of hostile actions against Iran," said Iran foreign ministry spokesman Ramin Mehmanparast.

"We are seeking methods to neutralize the new pressure," he said, quoted by Mehr news agency.

"We could double the trade volume with the countries importing our oil... (for example) we export five, 10, or 20 billion dollars' worth of oil, and instead we can receive the needed goods."

The Treasury stressed that the tightening of Iran's access to foreign trade and foreign exchange would not apply to farm commodities, food, medicine or medical devices

Tehran would be able to tap its accounts held by a bilateral trade partner to purchase such "humanitarian goods" from a third country if necessary.

The United States also placed sanctions on Islamic Republic of Iran Broadcasting, the Communications Regulatory Authority, and the Iranian Cyber Police, citing their extensive efforts to censor news and information flows as well as broadcasting forced confessions of political detainees.

"All of these entities have been involved in the Iranian government's ongoing attempts to shut their population off from the world through various forms of censorship and intimidation," the administration official said.

The sanctions forbid US citizens and entities from doing business with those groups, and lock up any assets they might hold in US jurisdictions.

The new US sanctions came ahead of the fourth round of talks between Iran and six world powers -- the United States, Britain, France, Russia, China and Germany -- in Kazakhstan on February 26 on Tehran's nuclear activities.

US State Department spokeswoman Victoria Nuland said Wednesday that Washington hopes for concrete progress in the talks.

"Our hope is that after applying the toughest sanctions we've had in international history... that this round will offer a real opportunity for Iran to discuss substance," she said.