MEXICO CITY, March 16 (Reuters) - Greater quantities of synthetic opioid fentanyl directly enter the United States and Canada than Mexico, Mexico's president said on Thursday, pushing back against U.S. criticism of his record on cracking down on trafficking in the lethal drug.

A powerful painkiller, fentanyl has been blamed for fueling a surge in U.S. drug overdoses, and some Republican lawmakers have urged Washington to authorize the use of military force in Mexico to bring the country's drug gangs to heel.

Last month Anne Milgram, head of the U.S. Drug Enforcement Administration (DEA) told a U.S. congressional hearing on drug trafficking that Mexico's Sinaloa Cartel and rival Jalisco New Generation Cartel were responsible for the "vast majority of fentanyl tHowever, President Andres Manuel Lopez Obrador told a news conference it was not Mexico that was responsible for the introduction of most fentanyl into the United States.

"I maintain that more fentanyl reaches the United States and Canada directly than reaches Mexico," he said.

Lopez Obrador, who has bristled at suggestions the U.S. could intervene in Mexico, said Mexican officials had explained to him that only blue fentanyl pills turned up in Mexico.

"Over in the United States they've got all colors and flavors," the president said.

Nevertheless, Lopez Obrador, who criticized U.S. efforts to deal with American drug traffickers, suggested there was some kind of fentanyl manufacturing in Mexico after the government earlier this week said it had "no record" of production.

Asked whether there were fentanyl production labs in the country, Lopez Obrador said "yes" but underlined that the raw materials used to make the drug were coming from Asia.hat is coming into the United States."