A threat by Donald Trump to pull the United States out of the global postal system could lead to a "nightmare scenario" of mail going undelivered, packages piling up and American stamps no longer being recognized abroad, the U.N. postal agency said.

The Universal Postal Union (UPU) has been holding an emergency meeting in Geneva to persuade Washington not to follow through on a threat to quit the agency, which sets rules to ensure mail gets delivered around the globe.

The Trump administration says it wants to charge other countries more than UPU rules now permit to have letters and packages delivered in the United States. It has set a deadline of next month for rates to be raised or it will quit.