Amazon.com posted quarterly net profits that have more than tripled, sending its share price up over 20%.

Amazon's shares had already seen a 55% increase after they revealed improved earnings in the first quarter, overall their stock has more than doubled in the last year.

"It's an amazing performance", Bernstein Research analyst Jeffrey Lindsay told Reuters.

"A lot of it seems to have come in the United States, which is particularly surprising since that's considered Amazon's most mature market."

Net income in the second quarter rose to $78 million compared with $22 million, a year earlier.

Amazon's chief financial officer, Tom Szkutak, claimed the US success was due to to "Amazon Prime" a scheme that charges $79 a year for unlimited 2-day shipping.

"Subscribers are increasing, customers are buying more, it's becoming a more meaningful part of our overall units."

"It's certainly impacting the growth you're seeing this quarter", Szkutak told reporters.

Their business model of offering a marketplace for third party sellers was also mentioned as a factor of their success.

Operating income for the year is now predicted to range between $540 million and $640 million, above previous estimates of $463 million to $593 million.