White House national security adviser Jake Sullivan said on Sunday the United States had begun to communicate with Iran over the country’s detention of American citizens, calling the matter a “complete and utter outrage,” Reuters reported.

Iran has arrested dozens of dual nationals, including several Americans, in recent years, mostly on espionage charges. Rights activists accuse the country of trying to use the detentions to win concessions from other countries, though Tehran dismisses the charge.

“We have begun to communicate with the Iranians on this issue,” Sullivan told CBS News when asked if the administration had started hostage negotiations with Iran.

Sullivan added that US President Joe Biden was determined to prevent Iran from getting a nuclear weapon and that diplomacy was the best way to do that.

The United States said last week it was ready to talk to Iran about both nations returning to a 2015 accord abandoned by former President Donald Trump’s administration that aimed to prevent Tehran from acquiring nuclear weapons while lifting most international sanctions.

“Iran has not yet responded,” Sullivan said.