The Obama administration on Thursday imposed new sanctions against Iran’s largest air carrier, accusing it of aiding government organizations that support international terrorism and nuclear proliferation.

The new measures announced by the Treasury Department allege links between Iran Air, the country’s national airline, and illegal weapons shipments to terrorist groups in Syria, and also to the transport of high-tech parts for Iran’s advanced missiles and nuclear programs.

The sanctions restrict U.S. firms from conducting business with the airline in the United States or overseas. Also targeted for sanctions was Tidewater Middle East Co., a major port operator in Iran.

U.S. officials said the measures were indirectly aimed at Iran’s powerful Islamic Revolutionary Guard Corps, whose leaders are alleged to dominate the country’s illicit trade in weapons parts and technology. A statement attributed to Secretary of State Hillary Rodham Clinton and Treasury Secretary Timothy Geithner said the United States would seek to “sharpen the choice for Iran’s leaders to abandon their dangerous course.”

“Until Iran is prepared to engage seriously with us . . . we will continue to increase the pressure,” the statement said.

Iran Air, a commercial airline with a fleet of 40 aircraft serving 25 international cities, has been under a variety of U.S. and international sanctions for more than 15 years. Its jets are banned from many European countries, in part because of concerns about the airline’s safety record.

Tidewater, which operates in seven Iranian ports and manages a major terminal at the port hub Bander Abas, is owned by the Revolutionary Guard and has been previously accused of using its facilities for illegal shipments.

The sanctions are intended to “further expose the [Guard’s] central role in Iranian illicit conduct . . . so that the international community can take steps to protect against the risk of doing business” with the organization, a senior administration official told reporters in describing the measures at a news conference.

The official called the Revolutionary Guard “central to Iran’s nuclear and ballistic missile programs, its support for terrorism, its human rights abuses and its role in fomenting instability in the region.”

Separately, the United Nations’ former chief nuclear inspector for Iran warned of a possible nuclear arms race in the Middle East if Iran is permitted to acquire nuclear weapons.

Olli Heinonen, who until last year headed the safeguards department for the International Atomic Energy Agency, told a congressional committee that Iran’s nuclear program was proceeding despite the recent Middle East turmoil and “maybe even accelerated.”

“This risk of proliferation triggered by an Iranian success in achieving nuclear weapons is an enormous, enormous red line to cross, and we should not underestimate it,” Heinonen said. “It’s not simply a question of one country getting nuclear weapons. It’s a risk of half a dozen nuclear weapon states in the Middle East in very short order thereafter.”

Iran insists that its nuclear ambitions are peaceful.