President Trump warned Friday that Iran could face "new and serious consequences" if it doesn't free U.S. citizens imprisoned in Iran.

"President Donald J. Trump and his administration are redoubling efforts to bring home all Americans unjustly detained abroad," the White House said in a statement. "The United States condemns hostage takers and nations that continue to take hostages and detain our citizens without just cause or due process. President Trump is prepared to impose new and serious consequences on Iran unless all unjustly imprisoned American citizens are released and returned." It did not detail what those consequences could be, however.

Trump specifically called for the release of American citizens Robert Levinson and Baquer and Siamak Namazi.

Levinson is a former FBI agent who disappeared from Iran's Kish Island in 2007.

The Associated Press reported that his family met with State Department officials in Washington on Thursday to discuss Levinson's case. Some U.S. officials believe Levinson is no longer alive, the AP said.

On Oct. 18, Iran sentenced Baquer Namazi, 81, and his youngest son, Siamak, 45, to 10 years in prison for collaborating with a foreign government, the United States.

Baquer Namazi is a retired UNICEF official.

The U.S. government and the Namazi family dismiss the charges as baseless and consider their detention as a play by Iran's notorious Islamic Revolutionary Guard Corps to taunt America and extract concessions.