WASHINGTON (AP) — The U.S. Postal Service is seeking to increase the price of its first-class stamp by 5 cents to 55 cents to help stem its mounting red ink.

If approved by regulators, the 10 percent increase to the cost of mailing a 1-ounce letter would be the biggest since 1991. The price of each additional ounce would go down, from 21 cents to 15 cents.

The proposed increase would take effect in January. It comes as President Donald Trump has criticized the Postal Service for "losing a fortune" by not charging higher shipping rates for online retailers such as Amazon.com.

The Postal Service has seen years of financial losses as an unrelenting drop in mail volume and costs of its health care and pension obligations outweighed strong gains in package deliveries.