WASHINGTON — Diplomats say sunset clauses that will eventually ease limits on Iran’s enrichment capacity are proving the most difficult issue in US-European consultations to see if they can find a compromise to meet US President Donald Trump’s demands to toughen Iran policy.

Trump vowed in January that he would not waive US sanctions on Iran again if his demands to “fix” the Joint Comprehensive Plan of Action (JCPOA) and other non-JCPOA issues were not met.

US and European diplomats, who met in Paris last week, said there was some progress after two rounds of talks to try to work out a compromise. Teams from the United States and the three European powers that helped negotiate the 2015 Iran nuclear deal — Britain, France and Germany — plan to meet again as early as next week, as well as on the sidelines of the next meeting of the eight-member Joint Commission overseeing JCPOA implementation in mid-March, sources said.

Consultations of the US-E3 working group on Iran are intensifying with an eye to May 12, the next deadline by which Trump must decide if he will issue a waiver of relevant US sanctions on Iran to stay in compliance with the JCPOA.