(CNN) -- The United States on Thursday placed additional sanctions on what it called Iran's nuclear proliferation network.
The Treasury Department said 11 companies and four individuals were being from barred doing business with U.S. firms, and it publicly identified what it called front companies involved in Iran's oil trade. The department also identified 20 Iranian financial institutions whose assets are being blocked.
The sanctions are a continuation of actions taken against Iran since 2006 to target its nuclear and ballistic missile proliferation activities.
"Iran today is under intense, multilateral sanctions pressure, and we will continue to ratchet up the pressure so long as Iran refuses to address the international community's well-founded concerns about its nuclear program," Treasury Undersecretary David Cohen said in a news release.
Thursday's actions also were aimed at disrupting front companies that Iran uses to sell and move its oil, he said.
U.S. administration officials, briefing reporters in Washington on Thursday, said the front companies were based in Switzerland, Hong Kong and Malaysia and the United Arab Emirates, have names like Petro Suisse and Hong Kong Intertrade, and had "no obvious connection to the government of Iran."
Late last year the United States passed legislation forcing countries that purchased oil from Iran to significantly reduce their oil buying from that country. Many of the sanctions added Thursday are designed to prevent Iran from selling its oil through fake companies that other countries may not realize are actually run by the Iranian government because of the U.S. move.
Officials said they were also identifying the registration numbers of 57 oil tankers owned by Iran so oil importers around the world will know whether they are dealing with an Iranian tanker. According to a U.S. Treasury press release, those ships were previously identified by the U.S. government as being owned by the Iranian government and had since been repainted, renamed and reflagged to deceive potential customers.
"We are constantly in the process of assessing the effectiveness of our actions and considering additional steps that may be effective and to have our sanctions be even more potent," according to a senior administration official.
Western powers fear that Iran may be trying to build nuclear weapons, despite the country's insistence that its nuclear program is for peaceful purposes. The West has been using sanctions and diplomacy to try to stop Iran from producing nuclear arms.
The sanctions include an oil embargo by the European Union.
The U.S. officials said the continued sanctions were having a significant impact on Iran's oil sales but could not offer exact details.
The sanctions by the United States and international partners have "significantly impaired Iran's access to the international financial system, driving down the value of its currency and limiting Iran's ability to earn and access revenue from oil sales," according to the administration officials.