London — Advocates for Americans imprisoned by Iranian authorities said on Friday they were concerned the Trump administration’s hard line on Iran would close off the chance for talks to secure the prisoners’ release.

In a major shift in US policy, President Donald Trump announced he would not certify that Iran is complying with a 2015 nuclear deal and warned that he might ultimately terminate the agreement.

The administration also designated Iran’s Revolutionary Guard Corps, the dominant player in the country’s security, economy and politics, as a terrorist group, a move one expert said would make the group less willing to negotiate over the prisoners.

Jason Rezaian, a Washington Post reporter who was detained by Iran for 18 months, said on Twitter that Trump’s Iran strategy “will only hurt American hostages being held in Iran,” Reuters reported.

The White House did not respond to a request for comment. A State Department official said the United States calls for the “immediate release” of US citizens held “unjustly” in Iran.

The American citizens who have been detained in the last two years in Iran are seven and hold a US permanent residency.

In January 2016, the Obama administration secured the release of five Americans imprisoned in Iran by agreeing to a much-criticized prisoner swap after protracted direct talks with Iran.