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Technology giant Apple has surpassed $600bn (£379bn) in value for the first time, affirming its position as the world's most valuable firm.

Apple shares reached a high of $644 on Tuesday, having only just crossed the $500 threshold back in February.

Apple's stock is up almost 60% since the start of the year.

It is another milestone for the iPhone maker, whose shares were worth as little as $3.19 in 1997 when it faced the possibility of bankruptcy.

Rival Microsoft is worth $260bn and in 1999, at the peak of the dotcom boom, reached $619bn in market capitalisation.

There has been some talk that there is another tech boom - and potential bust - under way.

On Monday, Facebook said it will pay $1bn to buy Instagram, the popular photo-sharing smartphone app, which has only 13 employees and was launched in October 2010.

Apple's resurgence

Apple's revival under Steve Jobs, who died last year, came about first in computers and then the iPod music player, which was followed by the iPhone and iPad.

Jobs co-founded Apple in Silicon Valley in the 1970s, but was fired in 1985.

He was asked to rejoin in 1997 and changed the company's product lines, culminating in the success of the iPhone and its tablet spin-off, the iPad.

In January, Apple reported record-breaking net profits for the last three months of 2011. At the end of last year, it revealed it had $97.6bn in cash.

In March, the tech giant under current chief executive Tim Cook said it will use its cash to start paying a dividend to shareholders and to buy back some of its shares.

It expects to use $45bn over the next three years.