WASHINGTON — The US Postal Service says it lost $1.9 billion over the last three months, and warns that losses will continue to mount without help from Congress.

The loss for the financial quarter ending March 31 was narrower than a $3.2 billion loss for the same period last year.

Nevertheless, the Postal Service continues to lose money at a rapid pace due to a decline in mail volume and a Congressional requirement that it make advance payments to cover expected health-care costs for future retirees.