Concern over Whitney Houston's health has grown after she pulled out of the second Michael Jackson gala concert at the last minute.

The singer looked emaciated when she sang at the first of the shows in New York.

She refused to say why she would not appear at Madison Square Garden for Jackson's second comeback concert.

Whitney sang for five minutes at the first concert on Friday but her appearance shocked the audience. The singer appeared to have lost an enormous amount of weight in the 18 months since she last performed in public.

Internet columnist Matt Drudge said: "Images of Houston completely wasting away into skeletonism - and appearing to be at stages of illness beyond even Karen Carpenter before she passed - still haunt."

ABC's Barbara Walters said Whitney looked almost like a skeleton.

Houston was charged with marijuana possession in January 2000 after half an ounce of the drug was found in her bag at a Hawaiian airport.

Security guards who searched her bag found the marijuana in two plastic bags and three partially smoked joints. But Houston and husband Bobby Brown caught their flight to San Francisco before police arrived.

She later pleaded no contest to a possession charge and was ordered to pay �2,100 to a youth-orientated anti-drug programme in place of community service.

Houston recently signed an extension to her recording contract with Arista records valued at more than �70m.