Fearful that an Israeli assassination effort would doom the negotiations, the United States, according to some of the officials, went so far as to ask other countries in the region to warn Iran about the possibility Israel could target the two officials.

U.S. officials acknowledged that during the intense phase of the war, Mr. Araghchi and Mr. Ghalibaf, as senior government officials, could have been legitimate targets for Israel, which was intent on toppling Iran’s hard-line government. But after the negotiations started in earnest in April, American officials believed that any attempt to kill the Iranian leaders would end the talks and reignite the fighting.