Consumer love for Apple's iPhones and iPads over the holiday season propelled the company's stock past US$500 for the first time, the company said on Monday.

Apple reported massive sales for its gadgets over the Christmas buying season as a key reason why its stock has surged over the past weeks.

Apple Inc. has been sparring with Exxon Mobil Corp. for title of the world's most valuable company since last summer. But the recent stock rally gives Apple a 17 per cent edge on the oil giant.

Apple now boasts a market capitalization of $465 billion compared to Exxon's $400 billion.

For the quarter that ended in December, Apple posted global sales of $13.06 billion, which is double the previous year's figure.

Meanwhile, Apple's annual net income for 2011 was 46.3 billion – a 73 per cent yearly gain.

But as Apple's stock continues to rise, so do questions about worker conditions in the company's global manufacturing plants, particularly in China.

Last month, the tech giant revealed a list of its suppliers and company brass announced that an independent monitoring group will be allowed to check into manufacturing practices.

The U.S-based Fair Labor Association will begin probing plants where iPhones and iPads are built, including a large facility at Foxconn City in Shenzhen, which employs hundreds of thousands of workers.

The sprawling manufacturing facilities resemble cities themselves, and allow plant managers to keep staff onsite and nearby to improve efficiency.

There were several suicides at the Shenzhen plant in 2010, including cases were workers jumped from the roof of on-site dorms. And in May of 2011, an explosion at a plant in Chengdu left three people dead and 15 injured.

The furor over Apple's Chinese suppliers was further flamed by a January New York Times story that reported long working hours and allegations that employees were woken up in the middle of the night to pull long shifts.

The article also described long hours of mandatory overtime and crowding in worker dorms and unsafe plant conditions.

Though Apple has been checking its own plants outside of the U.S., it is the first tech company to become a member of FLA.

FLA's audits are expected to probe facilities where about 90 per cent of Apple's manufacturing takes place.

It's expected that FLA's findings will be released in early March.

With reports from The Associated Press