Apple has forecast a strong Christmas quarter after a better than expected 16% jump in iPhone sales, and the strongest growth in Mac computer shipments in years helped the company surpass Wall Street's targets. 

The company last night projected stronger-than-expected revenue of $63.5 billion to $66.5 billion in the December quarter.

But sales of the iPad, which helped launch the mainstream tablet market in 2010, slid for the third quarter in a row. 

A gradual decline in tablet demand worldwide has worried investors already concerned with Apple's slowing growth, who are awaiting a new device that can energise its expansion. 

Sales of Apple's tablet slid more than 7% from the previous quarter to 12.3 million units, and were down 13% from the same time last year. 

Some investors hope that Apple's recently forged alliance with International Business Machines, intended to drive tablet and phone sales to corporate customers, may help reverse a decline in sales of the tablet device.

Apple's chief financial officer Luca Maestri said in an interview that the two tech giants had already signed on 50 "foundational" or initial clients, and the two intend to introduce their first jointly designed software apps next month.

"The level of interest from the corporate world has been incredible," Maestri said. 

Apple's fortunes, however, still largely hinge on the iPhone, which accounts for half its business, and the company's ability to again re-define markets with new technology gadgets. 

The Apple Watch, the company's entry in the nascent wearables category, will not hit shelves till 2015. 

Orders for the iPhone 6 and 6 Plus began in September, helping Apple chalk up a 12.2% jump in revenue to $42.12 billion. That exceeded the roughly $39.9 billion that Wall Street analysts had predicted, on average. 

The new iPhone, which comes in larger sizes and a heftier price tag, marks Apple's best product launch on record. 

That helped push overall smartphone sales to 39.27 million in the September quarter, beating the roughly 38 million some on Wall Street had anticipated. 

Maestri said the new iPhones, which went on sale just last week in China, had already surpassed the previous-generation model in terms of volume. 

The company also posted the strongest percentage growth in Mac sales, of 21% to 5.5 million units, since the December quarter of 2011.