Iran considers any new U.S. sanctions on its ballistic missile program illegal and will respond accordingly, an Iranian Foreign Ministry spokesman said Thursday, raising fresh doubts about the implementation of a landmark nuclear accord reached in July.

“Such actions are unilateral, arbitrary and illegal and the Islamic Republic of Iran has warned the U.S. in this respect,” said spokesman Hossein Jaber Ansari, according to the official Islamic Republic News Agency.

Mr. Ansari didn’t elaborate on how Iran planned to respond.

The Obama administration is planning new missile-related sanctions on Iran, targeting almost a dozen companies and individuals in Iran, Hong Kong and the United Arab Emirates, The Wall Street Journal reported on Wednesday.

They would be the first new sanctions on Iran since six world powers, including the U.S., reached a landmark nuclear deal with Tehran in July.

That deal places restrictions on Iran’s nuclear program, including decommissioning thousands of uranium enrichment centrifuges and the removal of the reactor core at the Arak nuclear facility near Tehran, in exchange for relief from sanctions that have crippled its economy.

Western countries have long suspected Iran of trying to develop nuclear weapons. An International Nuclear Energy Agency report this month concluded that Iranian scientists had done research into such weapons until 2009.

Iran conducted two separate ballistic missile tests in October and November, sparking concern in the international community and a warning from the U.S. that it could impose new sanctions on the program.

A United Nations panel determined this month that those tests constituted a violation of U.N. resolutions aimed at preventing Iran from developing nuclear warhead-capable missiles.

Mr. Ansari said on Thursday that Iran’s missile program was for defensive purposes, and was unrelated to the nuclear deal, as long been claimed by Iranian officials.

“There is nothing to prevent Iran from pursuing its legitimate right of reinforcing its defense strength and national security,” he said.