U.S. President Barack Obama on Friday called on Iran's government to immediately release three detained Americans - Saeed Abedini, Amir Hekmati and Washington Post reporter Jason Rezaian - and to help find Robert Levinson, an American who disappeared in Iran eight years ago, the White House said.

Obama made the statement in conjunction with Nowruz, the Iranian New Year. "Today, as families across the world gather to mark this holiday, we remember those American families who are enduring painful separations from their loved ones who are imprisoned or went missing in Iran," he said in a statement.

Washington and Tehran severed relations after Iran's 1979 Islamic Revolution. The United States and other world powers are currently engaged in sensitive negotiations with Iran aimed at curbing its nuclear program in exchange for easing economic sanctions.

Abedini, an Iranian-American Christian pastor from Boise, Idaho, was sentenced to eight years in prison in 2013 by an Iranian court for undermining Iran's national security by setting up home-based Christian churches in Iran from 2000 to 2005.

"He must be returned to his wife and two young children, who needlessly continue to grow up without their father," Obama said.

Hekmati, a former U.S. Marine from Flint, Michigan, was arrested in August 2011 while visiting his grandmother in Tehran, and convicted of spying for the U.S. Central Intelligence Agency, a charge his relatives and the U.S. government deny.

"His family, including his father who is gravely ill, has borne the pain of Amir's absence for far too long," Obama said.

Rezaian of Marin County, California, an Iranian-American dual citizen and the Washington Post's bureau chief in Tehran, has been detained for nearly eight months on what Obama called "vague charges."

"It is especially painful that on a holiday centered on ridding one’s self of the difficulties of the past year, Jason’s mother and family will continue to carry the heavy burden of concern regarding Jason’s health and well-being into the new year," Obama said.

Robert Levinson, a private detective and ex-FBI agent, Levinson disappeared in 2007 from Kish Island, an Iranian resort in the Gulf. Current and former U.S. officials have acknowledged that Levinson was a source for the CIA when he disappeared.

"His family has now endured the hardship of his disappearance for over eight years," Obama said.

The United States has offered $5 million for help in finding Levinson.