The United States on Tuesday announced it is imposing sanctions against seven foreign firms -- including from Venezuela, Singapore, and Israel -- as part of efforts to check Iran's nuclear ambitions.

It also imposed sanctions against 16 other foreign firms and individuals, including from China, over trade with Iran, Syria and North Korea in goods or technology that may be used for weapons of mass destruction or missiles.

The US State Department said it was targeting the seven foreign firms as part of a growing squeeze on Iran's energy sector whose revenues it said are used to fund a nuclear program suspected of developing weapons.

Deputy Secretary of State James Steinberg said all seven were involved in the supply of refined petroleum products, including gasoline to Iran. Two firms were previously hit with sanctions for similar reasons.

"Iran has lost millions in potential revenue by converting petrochemical plants to produce gasoline to make up for the dramatic shortfall in gasoline imports," the State Department said in a statement.

The seven firms include Petroleos de Venezuela (PDVSA), as well as PCCI (Jersey/Iran), Royal Oyster Group (UAE), Speedy Ship (UAE/Iran), Tanker Pacific (Singapore), Ofer Brothers Group (Israel), and Associated Shipbroking (Monaco).

"The sanctions we have imposed will cut off PDVSA's access to US government contracts, US import-export financing and licenses for controlled technologies," Steinberg said.

"These sanctions will not prevent PDVSA's sale of oil to the United States or other markets, and the sanctions do not affect the operations of PDVSA's subsidiaries," he said.

"Today's action adds further pressure on Iran to comply with its international obligations," Steinberg said.

"By imposing these sanctions, the secretary sends a stern and clear message to companies around the world: those who continue to irresponsibly support Iran's energy sector and help facilitate Iran's efforts to evade US sanctions will face serious consequences," Steinberg said.

US senators on Monday unveiled a bill that would boost enforcement of existing sanctions, bring fresh pressure on Iran's oil sector and military, and shine a spotlight on the regime's poor human rights record.

The legislation, called the Iran, North Korea and Syria Sanctions Consolidation Act of 2011, aims to increase pressure on companies still doing business with Iran's energy industry, notably Chinese firms that are on existing US lists of violators but have not been sanctioned.

The European Union on Monday tightened its own sanctions on Iran, adding more than 100 firms to a blacklist of companies hit by an assets freeze amid efforts to revive international talks to make Tehran halt its nuclear program.

Steinberg said the United States is also imposing sanctions against 16 foreign entities and individuals from China, Belarus, Iran, North Korea, Syria and Venezuela as part of an effort to curb weapons proliferation.

He said those sanctions were imposed "pursuant to the Iran, North Korea, Syria non-proliferation act... for their activities in support of WMD (weapons of mass destruction) and missile programs."

He said the 16 were targeted for the "transfer to or acquisition from North Korea, Syria or Iran of goods, services or technologies controlled under the various export control regimes or otherwise had the potential to make a material contribution the developments of WMD or cruise or ballistic missile systems."

Most of them were targeted for "proliferation activity involving Iran," he added.

The sanctioned entities and individuals are:

-- Belarusian Optical Mechanical Association and BelTechExport (Belarus)

-- Karl Lee, Dalian Sunny Industries, Dalian Zhongbang Chemical Industries Company, Xian Junyun Electronic (China)

-- Milad Jafari, Defense Industries Organization, Islamic Republic of Iran Shipping Lines (IRISL), Islamic Revolutionary Guards Corps Qods Force, SAD Import-Export Company, and Shahid Bakeri Industries Group (SBIG) (Iran)

-- Tangun Trading (North Korea)

-- Industrial Establishment of Defense and Scientific Studies and Research Center (SSRC) (Syria)

-- Venezuela Military Industries Company (CAVIM)