Mexican drug cartels are strengthening alliances with a Colombian terrorist group that has engaged in a decades-long war to topple the U.S.-supported government in Bogota, intelligence officials said.

The cartels are aggressively negotiating with the Marxist guerrilla group Revolutionary Armed Forces of Colombia, better known as FARC for the purchase of “multiton quantities of cocaine directly from South America,” said Michael Braun, former administrator and chief of operations at the U.S. Drug Enforcement Administration.

The strengthened ties between Mexican cartels and the FARC, which for years offered security for Colombian drug organizations in return for cash for weapons, is a bad sign for already tenuous U.S. border security, Braun said.