WASHINGTON, April 12 (Reuters)

Senior British, French and German officials believe they are making headway towards an agreement that would address U.S. President Donald Trump's concerns about the Iran nuclear deal, a European diplomat said on Thursday.

"We came out feeling like we are making good progress towards addressing the president's concerns and coming (up) to an agreement," the diplomat told a small group of reporters on condition of anonymity.

Senior diplomats from the three European nations met Brian Hook, the U.S. State Department's director of policy planning, in Washington on Wednesday to try to find a way to salvage the arms control pact.

The crux of the 2015 agreement between Iran and six major powers - Britain, China, France, Germany, Russia and the United States - was that Iran would restrict its nuclear program in return for relief from sanctions that have crippled its economy.

On Jan. 12, Trump delivered an ultimatum to Britain, France and Germany, saying they must agree to "fix the terrible flaws of the Iran nuclear deal" or he would refuse to extend the U.S. sanctions relief on Iran that it calls for. U.S. sanctions will resume unless Trump issues new "waivers" to suspend them on May 12, although it is unclear how fast they would go into effect.