China threatens retaliation over expanded Iran sanctions

LATEST: China says it will retaliate if the U.S. expands secondary sanctions on Iran to include Chinese companies.
The warning raises the risk that Washington's campaign against Tehran could spill further into U.S.-China economic relations.
China remains Iran's most important oil buyer, making Chinese companies a key target if Washington attempts to tighten enforcement around Iranian energy exports.
The next question is whether sanctions remain focused on Iran or evolve into another source of direct economic friction between the world's two largest economies.