Richard Grenell, the United States ambassador to Germany, ripped Iranian Foreign Minister Javad Zarif after he appeared to complain about the State Department prohibiting Zarif from visiting a sick colleague during his visit to New York City for the United Nations General Assembly.

On Saturday, Zarif tweeted: "Thanks to technology, I was able to see and talk to my friend of 40 years and our UN ambassador Ravanchi, who is in hospital here in New York only a few blocks away." The State Department said Friday that Zarif could visit Majid Takht-Ravanchi, Iran's ambassador to the United Nations, in person if Tehran released an American hostage.

"Iran has wrongfully detained several U.S. citizens for years, to the pain of their families and friends they cannot freely visit," the State Department said. “We have relayed to the Iranian Mission that the travel request will be granted if Iran releases a U.S. citizen."

Grenell, a staunch critic of the Iranian regime, lashed out at Zarif, tweeting: "You deny the Iranian people access to this US technology and social media sites. And you continue to take hostages. Your hypocrisy knows no bounds."

US: IRANIAN FOREIGN MINISTER CAN ONLY VISIT SICK COLLEAGUE ON NY TRIP IF IRAN RELEASES HOSTAGES

In a follow-up tweet, Grenell asked Zarif: "Where’s Bob Levinson? For 12 years he’s been a US hostage in Iran. The longest held US hostage." The ambassador quoted a tweet from an account run by Levinson's children, who wrote to Zarif: "It's nice that you recognize how important it is to be able to see and speak with the ones you love. We are still waiting to see and talk to our father Bob Levinson, held by your country with no human rights."

Levinson has been missing in Iran since 2007. Four other Americans are known to be imprisoned in that country: U.S. Navy veteran Michael White, Princeton University Ph.D. candidate Xiyue Wang, Siamak Namazi and his father Baquer.

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Asked Thursday about Americans imprisoned in Iran and Iranians imprisoned in the United States, Iranian President Hassan Rouhani told Fox News that the Trump administration had said that if at least one American was freed by Iran, then Washington would reciprocate.

Rouhani cited the June release of Nizar Zakka, a Lebanese and permanent U.S. resident detained in 2015 and sentenced to 10 years in prison on accusations of spying for the United States. “They only thanked Iran,” Rouhani said. “The ball stands in America’s court.”

The Associated Press contributed to this report.