The United States stepped up pressure on Iran over its nuclear program on Thursday, imposing sanctions on seven companies and five individuals, including Iran's atomic energy chief.

The U.S. Treasury Department said the action would bar those companies and individuals from doing business with U.S. firms or citizens and freeze any assets they have in the United States.

Among those subject to the new sanctions is Fereydoun Abbasi-Davani, head of Iran's Atomic Energy Organization and a survivor of an assassination attempt that Iranian officials have blamed on Israel. At least four scientists associated with Iran's nuclear program have been killed since 2010; Abbasi-Davani was wounded.

Iran has accused Israel and the United States of plotting the killings to set back its nuclear program. Washington has denied any U.S. role and Israel has declined to comment.

Foreign banks that handle transactions for the companies and individuals listed by the Treasury Department could also lose their access to the U.S. banking system under the U.S. sanctions regime.

"The U.S. will target those involved in Iran's illicit enrichment activities," Undersecretary of the Treasury David Cohen said in a statement. "The entire world has made it clear that these activities are unacceptable."

The United States has said Tehran's nuclear program is aimed at acquiring atomic weapons, a charge Iran has repeatedly denied.