"The macro-economic impact would be serious, not only if the United States took action, but especially if other countries were to retaliate, notably those who would be most affected, such as Canada, Europe, and Germany in particular," Lagarde said in an interview with French radio RTL.
"If world trade were jeopardized by such measures (tariffs), they would become a vector for lower growth and a slowdown of commerce. The impact on growth would be a formidable," she said.
Earlier this month, US President Donald Trump announced duties of 25 percent on steel and 10 percent on aluminum, a move that triggered global outcry and forced world powers to take counter-measures to reject what they said economic nationalism.
Noting that the White House had not thought enough about the risk of imposing new import duties, the former French Economy Minister warned of a possible trade-war.
"In a so-called trade war, driven by reciprocal increases of import tariffs, nobody wins, one generally finds losers on both sides," she said.