Image caption The first 20 articles read each month will remain free

The New York Times has said it will start charging North American users for some of its online content.

The newspaper will initially charge readers in Canada, with its US audience facing charges from 28 March.

Different charges will apply depending on whether readers use a smartphone, a computer or an iPad.

The newspaper hopes to make up for lost revenue as newspaper sales have fallen and as advertising earnings have slipped.

"Our decision to begin charging for digital access will result in another source of revenue, strengthening our ability to continue to invest in the journalism and digital innovation," said chairman and publisher Arthur Sulzberger Jr.

Different charges

The newspaper is eager not to alienate its existing readers online, so each month the first 20 articles will be free.

Readers will then be charged:

$15 (£9) per month for unlimited access to the website and its smartphone app

$20 per month for unlimited access to the website and an iPad app

$35 per month for unlimited access to the website, a smartphone app and an iPad app.

Subscriptions to the print edition of the newspaper will include full access to the internet content.

"We need to make sure that part of our business continues to grow," said Martin Nisenholtz, senior vice president of digital operations at the company.

"The way we think we can do that is taking this metered approach charging the folks who are drinking deeply."

The New York Times tried to introduce charges for its online content in 2005, but the effort was not successful.

The Financial Times (FT), however, introduced a similar charging system three years ago.

The FT said "both digital content and digital advertising revenues" have risen every year since.

"We've found our readers seek out and, importantly, pay for... responsible and authoritative journalism that they can trust," said Rob Grimshaw, managing director, FT.com.

Shares in New York Times rose 4.3% in early trading.