WASHINGTON — The U.S. opened a new front in its mounting economic conflict with China, starting a process to withdraw from a 144-year-old international postal body whose discounts allow Chinese merchants to ship small packages to U.S. customers at sharply lower rates.

In announcing the move by the State Department, senior White House officials said Wednesday that the U.S. will go ahead with a threat to set its own “self-declared” rates for packages from abroad.