U.S. President Donald Trump's sweeping foreign aid freeze has stalled a United Nations program in Mexico aimed at stopping imported fentanyl chemicals from reaching the country's drug cartels, according to eight people familiar with the situation.

Donald Trump’s freeze on foreign aid is hurting one of his major campaign promises: taking on fentanyl and ending overdose deaths from the drug in the U.S. Reuters reports that a United Nations counternarcotics program that seeks to stop fentanyl chemicals from reaching Mexico’s drug cartels has been held up by Trump’s aid freeze,