Nine Americans killed in Mexican ambush, Trump urges joint war on drug cartels

"This is the time for Mexico, with the help of the United States, to wage WAR on the drug cartels and wipe them off the face of the earth," Donald Trump said in a tweet reacting to the massacre.

Later, he and Lopez Obrador spoke by phone, with the U.S. president offering help to ensure the perpetrators face justice. The Mexican leader said he would ensure justice was done.

Prior to the call, Lopez Obrador rejected what he called any foreign government intervention.

Mexico has used its military in a war on drug cartels since 2006. Despite the arrest or killing of leading traffickers, the campaign has not succeeded in reducing drug violence and has led to more killings as criminal groups fight among themselves.

The government has registered more than 250,000 homicides in the last dozen years, most of them related to the drug war.