SAN FRANCISCO, April 26 (Xinhua) -- U.S. top online retailer Amazon announced Thursday a 20 percent price hike for its Amazon Prime membership in the United States, the first in four years, after it reported robust first-quarter earnings of 1.6 billion U.S. dollars earlier in the day.

During a call with investors on Thursday, Amazon said the new Prime rate for U.S. users only will rise from 99 U.S. dollars a year to 119 dollars. The new price is set to apply to new subscribers on May 11 and to existing members on June 16.

The latest fee increase is supposed to reflect the higher investments that the Seattle-based e-commerce company is making in the Prime program.

During the call, Amazon CFO Brian Olsavsky said that Amazon is seeing "rises in cost" for providing Prime services, which include shipping perks and video streaming.

Amazon CEO Jeff Bezos said earlier this month that his company has more than 100 million Prime members across the globe, as the number of Prime subscribers has surged in recent years.

The last price hike in Prime membership was made in March 2014, when Amazon's Prime member fees jumped from 79 dollars annually to 99 dollars.

Also on Thursday, Amazon said in its Q1 financial report that its earnings for the first months of 2018 more than doubled to 1.6 billion dollars over the same period last year.