A US journalist who was imprisoned in Iran for 18 months filed suit Monday against the Tehran government, claiming his detention was aimed at "extorting concessions" from the United States.

The lawsuit filed in US federal court by Washington Post reporter Jason Rezaian and his family seeks unspecified damages from Iran for, among other things, "cruelty, torture, and abuse" during his captivity.

The Islamic Republic of Iran and the Islamic Revolutionary Guard Corps are named in the complaint "for severe personal injuries and other irreparable harm" suffered due to "unlawful acts of terrorism, torture, hostage taking" and other mistreatment.

Rezaian was among four prisoners freed in Iran in January, hours before world powers sealed a deal with Tehran on its nuclear program as part of an effort toward normalizing relations. In return, the US pardoned seven Iranian prisoners and dropped charges against 14 other Iranians.

According to the lawsuit, the Iranian government "targeted and arrested Jason Rezaian, subjected him to torture and other cruel treatment, and held him hostage for the unlawful purpose of extorting concessions from the US government and others."

His captors "tortured and tormented Jason using a cruel combination of harsh physical mistreatment and extreme psychological abuse," it added.

They also "threatened to maim and kill his wife Yeganeh," who was imprisoned for over two months as well, the complaint said.

Rezaian's lawsuit said he was accused of espionage and other offenses by Tehran "in a futile effort to justify its crimes," but that these were "blatant lies" and part of an effort to increase his value as part of an exchange with the United States.

"In reality, Jason committed no crime and was never legitimately tried, convicted, or sentenced -- even according to Iranian standards," the lawsuit said.

The family is seeking damages under the Foreign Sovereign Immunities Act, which generally does not allow Americans to seek damages from foreign governments but makes exceptions for "terrorism."