WASHINGTON, Aug 21 (Reuters) -

The United States on Friday said it was imposing visa restrictions on 13 Iranian officials it accused of involvement in "gross violations of human rights" for a 1990 assassination of an Iranian opposition figure in Switzerland.

The U.S. State Department did not name the 13, but in a statement said it was also designating a 14th Iranian, Hojatollah Khodaei Souri, who it said as director of Iran's Evin Prison ran an institution "synonymous with torture and other cruel, inhuman, or degrading treatment or punishment."

"The United States will continue to pressure Iran to treat its own people with dignity and respect," U.S. Secretary of State Mike Pompeo said.

Pompeo's statement barring the 13 officials from traveling to the United States was likely in reference to the 1990 killing of Kazem Rajavi, a leading opponent of the Iranian Government who was shot near his home in Switzerland, Reuters reported at the time, citing relatives.

Rajavi was the brother of the leader of the People's Mujahedeen, a leftist guerrilla group. The Mujahedeen's European office in Paris at the time issued a statement quoting Massoud Rajavi as saying the Iranian Embassy in Switzerland organized the killing.

Pompeo said the officials, who he called "assassins," posed as Iranian diplomats and acted "under the highest orders of their government to silence opposition and show that no one is safe from the Iranian regime."

Tensions between Washington and Tehran have spiked since Trump unilaterally withdrew in 2018 from the Iran nuclear deal struck by his predecessor, Barack Obama, and began reimposing sanctions that had been eased under the accord.

The United States moved on Thursday to restore U.N. sanctions on Iran, including an arms embargo, arguing Tehran was in violation of the deal it struck with world powers in 2015 even though Washington itself abandoned the agreement.