US Secretary of State Mike Pompeo (Photo: AFP/VNA)

The four firms, accused of "transferring sensitive technology and items to Iran's missile program," will be subject to restrictions on US government aid and on their exports for two years, Secretary of State Mike Pompeo said in a statement.

The sanctions, imposed Wednesday, were against two Chinese-based companies, Chengdu Best New Materials and Zibo Elim Trade, as well as Russia-based Nilco Group and Joint Stock Company Elecon.

President Donald Trump withdrew the US in 2018 from the Iran nuclear deal and has since re-imposed crippling sanctions on Iran in what he calls a campaign of "maximum pressure” that would encourage Iran to return to the negotiating table.