The wealth of U.S. high-net-worth individuals should rise by 25 percent over the next three years, according to Timetric, a London-based research and analysis firm for the financial industry.

Although there was a downturn in the number of high-net-worth individuals after 2007, their numbers and wealth levels started to increase last year, Timetric says.

Timetric defines high-net-worth individuals as those with more than $1 million.

The U.S. high-net-worth population grew by 3.9 percent in 2012 over the previous year and its wealth rose 4.7 percent. The population will continue to grow by 19.4 percent by 2016, to 6.1 million individuals. Wealth will increase by 25 percent, to $23.5 trillion, Timetric says.

By 2011, the U.S. high-net-worth population accounted for 30 percent of the global high-net-worth population, according to the firm. But the U.S. number decreased 3.1 percent, from 5.3 million to 5.1 million, between 2007 and 2011.

Alternatives investments are expected to be the fastest growing asset class used by high-net-worth investors, followed by equities and fixed income, according to Timetric. There will be a movement away from cash and about a 20-percent drop in European investments over the next four years due to concerns about the euro, Timetric predicted.