US computer software giant Microsoft announced Monday that it would buy back another 40 billion dollars of its shares.

Microsoft, whose share price has shed nearly 30 percent this year, said in a statement that the buy-back would last until September 2013, and that it had completed its previous 40-billion-dollar stock repurchase program.

The Redmond, Washington-based company said its board of directors had also declared a quarterly dividend of 13 cents a share, an increase of 18 percent over the dividend the previous quarter.

"These announcements illustrate our confidence in the long-term growth of the company and our commitment to returning capital to our shareholders," said Chris Liddell, chief financial officer of Microsoft.

Standard and Poor's Ratings Services and Moody's Investors Service said meanwhile that they had assigned the highest 'AAA' corporate credit rating to Microsoft and that the outlook was stable for the software giant.

"The ratings on Microsoft reflect the company's excellent operating performance and financial profile," said Standard and Poor's credit analyst Philip Schrank.

Shares of Microsoft, which failed in a bid earlier this year to buy Yahoo, rose more than two percent in early trading in New York to 25.72 dollars.

Revenues of Microsoft, the top supplier of operating system software for personal computers, exceeded 60 billion dollars in fiscal 2008.