VIENNA—A senior Iranian official raised the possibility of improved relations with the U.S. if a nuclear accord is reached between Tehran and world powers this month, amid signs the negotiations were progressing slowly.

French Foreign Minister Laurent Fabius, among several top diplomats who joined the talks Thursday, said before leaving Vienna that he would return Sunday—indicating little chance of an agreement before next week.

“There is still some work to be done, and I expect to come back to the talks Sunday evening,” he said, adding he hopes then to “be in a situation to advance toward perhaps…a robust deal.”

The U.S. and Iran have been foes for almost 40 years, after radical Islamist students and political parties overthrew the pro-American Shah Mohammad Reza Pahlavi in 1979.

But senior Iranian and American diplomats have been locked in intense, face-to-face negotiations on the nuclear issue for nearly two years. And their countries have at times shared the same regional goals, such as combating Islamic State militants in Iraq and Syria.