Washington Post Foreign Editor Douglas Jehl, left, Jason Rezaian and Post Executive Editor Martin Baron at Landstuhl Regional Medical Center in Germany, where Rezaian and two other Americans freed by Iran were taken for medical checks. (The Washington Post)

A United Nations human rights body, responding to a petition filed last summer by The Washington Post, has concluded that Iran’s detention of Post journalist Jason Rezaian violated several provisions of international law, including by failing to inform him of charges against him and depriving him of his right to counsel and an open, impartial trial.

Rezaian was released Saturday after 18 months’ imprisonment as part of a prisoner deal between Iran and the United States.

The Working Group on Arbitrary Detention, part of the U.N. Human Rights Council, said it also would refer allegations of physical ill-treatment of Rezaian, who was held in solitary confinement for part of his detention, to the U.N. special rapporteur on torture. It recognized his right to compensation from the Iranian government.

Iran never responded to queries from the group, which is charged with investigating cases of arbitrary detention and allegations of illegal treatment of prisoners. It has no enforcement powers but petitions governments involved for information and recommends courses of action to its parent body.

The group’s conclusions, dated Dec. 3, have not been publicly released. They will be included in the group’s annual report to the U.N. Human Rights Council. A copy was obtained by The Post.

In a statement released by The Post, Publisher Frederick J. Ryan Jr. said: “We are pleased that the U.N. Working Group has recognized that Iran had no legitimate basis for arresting Jason, let alone imprisoning him for 18 months. Its opinion leaves no doubt that this was an arbitrary and unlawful detention of a journalist. We take comfort in knowing that Jason receives this news as a free man.”

The Post’s petition to the United Nations was part of a series of steps the paper took to publicize the Rezaian case and bring international pressure on Iran, even as the Obama administration worked privately to secure the release of Americans it asserts were unjustly imprisoned.

As part of the deal implemented Saturday, five Americans were freed, and seven Iranians charged with or convicted here of violating sanctions against Iran were given clemency.

Three of the Americans in Iran — Rezaian, Idaho pastor Saeed Abedini and Marine veteran Amir Hekmati — left Iran aboard a Swiss government jet, arriving Sunday at a U.S. military hospital in Germany, where they were reunited with their families. All are dual U.S.-Iranian nationals, as was a fourth freed prisoner who opted to stay in Iran. The fifth prisoner, a U.S. student held since December, left Iran on Saturday on his own.