Mexican President Andres Manuel Lopez Obrador revealed on Tuesday that he had sent a letter to Chinese President Xi Jinping, requesting assistance in controlling shipments of fentanyl entering Mexico.

The letter, dated March 22, was read by Lopez Obrador in defense of Mexico’s efforts to curb the supply of the synthetic opioid, which has been linked to a surge in overdose deaths in the United States. He stated, “We come to you, President Xi Jinping, not to ask for your support in the face of these rude threats, but to request that for humanitarian reasons, you help us control shipments of fentanyl that can be sent from China to our country.”

The Mexican president’s letter was prompted by a recent visit from U.S. lawmakers who suggested he speak with China to address the fentanyl threat. While the Chinese embassy in Mexico did not immediately comment on the letter, U.S. critics, particularly Republican lawmakers, have accused Mexico of failing to stop drug cartels from producing and trafficking the potent painkiller.

In his letter, Lopez Obrador asked President Xi for information on fentanyl shipments, including details on when, where, how much, and by whom the shipments were sent. He mentioned that nearly 1,400 clandestine labs mixing fentanyl with other substances were destroyed in Mexico last year, and that seven tonnes of the drug were seized. However, the president asserted that Mexico does not produce fentanyl and that only 30% of the drug consumed in the U.S. enters via Mexico.

U.S. officials dispute Lopez Obrador’s claim that fentanyl is not produced in Mexico, arguing that the opioid is mass-produced in the country using chemicals largely sourced from China. Republican Senator Lindsey Graham, who has been pressuring Mexico on this issue, announced on Monday that he would introduce legislation on fentanyl, including designating Mexican cartels as foreign terrorist organizations.

Mexican Foreign Minister Marcelo Ebrard defended Mexico’s efforts, saying that no other country in the world is doing as much against fentanyl trafficking to the United States as Mexico. Ebrard emphasized that countering the fentanyl threat was a top priority and noted that the vast majority of people arrested in the U.S. for trafficking fentanyl are Americans.