WASHINGTON (Reuters) - The number of Americans with high blood pressure is on the rise thanks in large part to growing rates of obesity, researchers said on Tuesday.

But increasing numbers of those with high blood pressure, also called hypertension, are getting the condition treated, researchers from the US government's National Institutes of Health wrote in the journal Hypertension.

High blood pressure can lead to stroke, heart attack, heart failure or kidney failure. It is sometimes called the "silent killer" because it has no symptoms, and many people have it for years without knowing it.

Data spanning six years through 2004 showed that 29 percent of US adults had high blood pressure, compared to 24 percent in the six-year period ending in 1994, the researchers said.

Another 30 percent of Americans in the most recent period had a condition called prehypertension with slightly elevated blood pressure levels that often worsens into full-fledged high blood pressure, the researchers said.

That means that only 41 percent of Americans had normal blood pressure levels, the researchers said.

"The percentage of the population with high blood pressure is going the wrong way — it's increasing," Dr. Jeffrey Cutler of the NIH's National Heart, Lung, and Blood Institute said in a telephone interview.

Some of the leading factors behind high blood pressure include being obese or overweight, not getting regular physical activity, smoking and too much salt in the diet.

"It's not a big surprise to the degree to which this is related to obesity. We've seen it coming," Cutler added.

The study showed blacks continued to have higher rates of high blood pressure than whites.

During the 1999-2004 period, 61 percent of those with high blood pressure were undergoing treatment for it and 35 percent had their blood pressure under control, the researchers said. A number of different drugs are used to treat hypertension.

The findings were based on data on a nationally representative sample of 16,351 US adults for 1988-1994 and 14,430 adults for 1999-2004.