WASHINGTON (AP) - The Trump administration on Wednesday (Dec 11) hit Iran with new sanctions that target several transportation firms as it continues its "maximum pressure campaign" against the Islamic Republic over its nuclear and ballistic missile programmes.

The sanctions target Iran's state shipping line and a China-based company that has been involved in delivering missile parts to Iran.

They also add a layer of new penalties to a previously sanctioned Iranian airline, Mahan Air, which is accused of sending weapons to Iranian proxies in Lebanon and Yemen, and three of its sales agents.

The moves were announced by the Treasury and the State Department and will subject foreign firms and governments that do business with the targeted entities to sanctions themselves, including a freeze on any assets they may have in United States jurisdictions.

"Today's designations put the world on notice that those who engage in illicit transactions with these companies will risk exposure and sanctions themselves," Secretary of State Mike Pompeo said.

Mr Alireza Miryousefi, a spokesman for Iran's United Nations mission, tweeted later on Wednesday: "These sanctions are directly targeting ordinary Iranians' livelihood, but they will not be cowed by pressures by any foreign power. And another step in the US' #economicterrorism against Iranians, who are never fooled by the US's crocodile tears."

Mr Pompeo announced the sanctions even as he expressed hope that a weekend prisoner swop with Iran could lead to a dialogue between Washington and Teheran over prisoners.

He called last Saturday's release of Princeton graduate student Xiyue Wang in exchange for an Iranian jailed in the US a "happier note" that could yield progress.

"I do hope the exchange that took place will lead to a broader discussion on consular affairs. We are working to use this as an opportunity to continue that effort," he said.

"I hope that it portends well. We have had some indication that that may be the case, but I don't want to overstate that, I don't want to give false optimism about that pathway.

"If we can find an opening and deliver these people back to their families and back to America, we will certainly do that," he said.

US authorities say Iran is holding one American national and five dual US-Iranian nationals.

They include Navy veteran Michael White, who is serving a 10-year espionage sentence, as well as environmentalist Morad Tahbaz, an Iranian with US and British citizenship also initially sentenced to 10 years in prison, and Iranian-American father and son Siamak and Baquer Namazi.

Former FBI agent Robert Levinson has been missing for 13 years since disappearing in Iran.

Iran says American authorities are holding about 20 Iranian nationals in jail and on Monday said it was ready for more prisoner swops with the US.