The number of women billionaires in the world is rising fast, with a growing number of them from Asia and self-made, Swiss bank UBS Group AG said in a study released on Tuesday.

There are nearly seven times more women billionaires than 20 years ago and their average wealth has nearly doubled to $4.3 billion - topping the average $4 billion net worth of male billionaires, the bank said.

UBS, which offers wealth-management services, tallied 1,347 billionaires from around the world in the study but declined to name any of them.

Men still accounted for the overwhelming majority of billionaires, but women made up 11 percent last year, up from 9 percent two decades earlier.

Asia has seen the strongest growth rates for female billionaires over the last decade, to 25 from three.

Just over half of the female Asian billionaires made their own money, UBS said, with many of the women hailing from China and Hong Kong and much of the wealth coming from real-estate investments.

UBS said only 7 percent of European women billionaires and 19 percent of the Americans were self-made.