The United States and Europe announced strengthened sanctions against Iran on Friday, increasing the number of blacklisted companies and individuals linked to the Iranian nuclear program, in coordinated actions that underscored the failure to resolve one of the most intractable international disputes.

Under the expanded sanctions, the Treasury Department in Washington blacklisted four companies and one person, and the European Union blacklisted 18 companies and one person. The Treasury said one of the four companies was also involved in supplying weapons to the military of Syria’s president, Bashar al-Assad, Iran’s Middle East ally, who has been fighting an insurgency for nearly two years.

The steps were announced against a backdrop of uncertainty about the possible resumption of talks between Iran and the group of six big powers, which have been stalled for half a year, over Iran’s uranium enrichment activities. Despite statements from both sides this month that they were ready to re-engage, a date and place have yet to be announced. Russian news services reported Thursday that the renewed talks might be scheduled in January.

The Treasury said the additions to the sanctioned category — Iran’s SAD Import-Export Company, Chemical Industries and Development of Materials Group, Doostan International Company, Marine Industries Organization and its director, Mustafa Esbati — would now be subject to the freezing of assets and a ban on doing business with Americans. In addition, under a two-year-old law of sanctions on Iran, any foreign financial institution that does business with them risks expulsion from the American market.