The European Union is to host talks in Brussels next week with the British, French, German, and Iranian foreign ministers on Tehran's nuclear program.
EU foreign affairs head Federica Mogherini will lead the talks on March 16, one day after U.S. Secretary of State John Kerry meets his Iranian counterpart, Mohammad Javad Zarif, in Switzerland.
The meetings are part of international efforts to persuade Iran to restrain its nuclear enrichment program in exchange for relief from economic sanctions.
Iran and six world powers have set an end-of-March target for a framework agreement and a June 30 deadline for a full deal.
In an open letter to Iranian leaders on March 9, a group of 47 Republican senators warned that any nuclear deal between U.S. President Barack Obama's administration and Iran may be short-lived unless it obtains congressional approval.
Zarif was quoted as saying the letter suggests that the United States is "not trustworthy."