US warns Iran over detained Americans

US President Donald Trump warned that Iran would face "new and serious consequences" unless all unjustly detained American citizens were released and returned, the White House said in a statement on Friday.

Trump urged Iran to return Robert Levinson, an American former law enforcement officer who disappeared more than 10 years ago in Iran, and demanded that Tehran release businessman Siamak Namazi and his father, Baquer.

The statement capped a week of rhetoric against Tehran. On Tuesday, Washington slapped new economic sanctions against Iran over its ballistic missile program and said Tehran's "malign activities" in the Middle East undercut any "positive contributions" coming from the 2015 nuclear accord.

Those measures signaled that the Trump administration was seeking to put more pressure on Iran while keeping in place an agreement between Tehran and six world powers to curb its nuclear program in return for lifting international oil and financial sanctions.

Iran on Friday vented frustration over the sanctions which it says "violate" the terms of the nuclear deal, raising its concerns at a meeting with major world powers in Vienna.

The regular quarterly meeting to review the deal heard, as Washington already confirmed earlier this week, that Iran is sticking to its side of the pact with the United States, Russia, China, Britain, France and Germany.

The parties said in a statement they welcomed "Iran's continued adherence to its nuclear-related commitments".

Friday's statement said Trump and his administration were "redoubling efforts" to bring back all Americans unjustly detained abroad.