Chinas prime minister warned on Friday that there can be no winners in a trade war, hours after new US tariffs on Chinese goods kicked in with Beijing retaliating immediately.

"A trade war benefits no-one because it hurts free trade and the multilateral process," Li Keqiang told a summit with 16 EU and Balkan countries in Sofia in remarks translated from Chinese to Bulgarian.

"If a country wants to raise tariffs, China will respond to defend itself," he said.

The US and China on Friday launched tit-for-tat tariffs on each other's imports, the opening shots in what Beijing called "the largest trade war in economic history" between the world's top two economies.