A container that recently arrived in Mexico from China was found to contain the synthetic opioid blamed for hundreds of overdose deaths every day in the US, Andres Manuel Lopez Obrador said.

“We already have the evidence,” said Lopez Obrador, who in March wrote to his Chinese counterpart Xi Jinping, asking for help curbing fentanyl flows.

China’s foreign ministry said in response that there was no illegal fentanyl trafficking between the two countries.

Speaking at his daily news conference, Lopez Obrador said that Mexico would ask “very respectfully” for China to inform it when the contraband leaves its ports, and if possible seize it.

According to the US drug enforcement administration, criminal groups export chemicals from China to Mexico where they are used to produce fentanyl that is smuggled across the US border.

Mexico has denied that fentanyl is manufactured in its territory.