A frail Steve Jobs made a brief appearance on Thursday after a prolonged medical leave to unveil the iPad 2, a thinner and lighter version of Apple's popular tablet, which goes on sale from March 11 in the US.

Welcomed with a standing ovation, the 56-year-old Apple chief executive, who has been on medical leave since January, made his surprise appearance at a company event in San Francisco.

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Wearing his signature black turtleneck and blue jeans, Jobs began the over 70-minute event by saying, "We've been working on this product for a while and I just didn't want to miss today. Thank you for having me."

The new iPad, available in black and white, costs the same as the first version -- $499 to $829.

It will ship in the US on March 11 and in 26 other countries, including Mexico, New Zealand and Spain, on March 25.

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It comes with front and rear cameras, a feature which was missing in the first model.

It has a new Apple-designed chip that performs standard calculations twice as fast as its predecessor and offers a nine-fold boost in graphics performance.

The two cameras are for taking photos and video chatting.

The battery life, the same as the original, is about 10 hours of usage and a month on standby.

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The iPad 2 is thinner -- 8.8 millimetres instead of the current 13.4 millimetres and weighs 1.3 pounds, compared to the 1.5-pound original.

In the US, the iPad 2 will work on AT&T and Verizon Wireless networks.

"The new iPad 2 is actually thinner than your iPhone 4," Jobs said.

The new iPad also has a new case, a 'smart cover', that magnetically attaches to the front of the iPad to create a screen protector.

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It automatically puts the machine into sleep mode when it is attached and is sleeker as compared to the case for the original device, which was bulky and interfered with the tablet's performance.

Jobs made no reference to his health issues during the event.

A pancreatic cancer survivor, Jobs had also undergone a liver transplant. In his absence, chief operating officer Tim Cook has been running day-to-day operations at Apple.

Apple had unveiled the first iPad in April last year and had sold about 15 million iPads till December, 2010, generating revenues a little shy of $10 billion.

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"We've never had a product that got off to that fast a start. We have 90 per cent of the market," Jobs said, adding, "Our competitors are just flummoxed. They went back to their drawing boards, tore up their designs."

If 2010 was the 'Year of the iPad," 2011 will be the 'the year of the copycats,' referring to the influx of tablet computers by Apple's rivals -- Samsung, Research In Motion, HP, Dell and Motorola.

Jobs also introduced a new accessory for the iPad through which it can be connected to high-definition televisions.

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The $39 part plugs into the iPad's charging port and connects to a cable.

Apple also introduced updates to the software that runs on the iPad, iPhone and iPod Touch devices.

The new system, iOS 4.3, includes support for Apple's video-chat programme FaceTime and will enable people to hold conversations between iPads, iPhones and Mac computers.

Jobs also announced that e-books published by the country's largest consumer book publisher, Random House, will be sold directly via the iBookstore.