Efforts by world powers to preserve a 2015 landmark nuclear deal with Iran grew more complicated Monday with confirmation that Tehran had breached the pact's limitations on stockpiles of low-enriched uranium. The announcement by Iran, later verified by the United Nations' nuclear watchdog, that it had followed through with its threat to increase the stockpiles raises the pressure on the countries — Russia, China, Germany, France and Britain, along with the European Union — trying to preserve the pact following the unilateral withdrawal of the United States last year. European signatories to the nuclear deal, known as the Joint Comprehensive Plan of Action, or JCPOA, have warned of consequences, but have not yet said what they might be.