LOS ANGELES — Three hikers are feared dead after being swept over a waterfall popular with visitors in California's spectacular Yosemite national park, officials said Wednesday.

The three, who were on a day trip with family and friends, plunged over Vernal Fall on Tuesday after climbing over a guardrail and entering rapids about 25 feet from the top of the waterfall.

Witnesses said several people urged them to get out of the river, "since it was flowing swiftly and extremely cold."

"The area is signed as a dangerous area, and the group had crossed a metal guardrail placed there to keep visitors away from the dangerous fast-moving water," said a park statement.

Yosemite, east of San Francisco, is one of the most visited national parks in the United States, and a top Californian attraction that draws hikers and daytrippers from around the world.

Water levels are currently higher than normal and streams and rivers running faster because of a relatively cool spring and summer, meaning that a heavy winter snowpack has taken longer to thaw and run downstream.

The Merced River which feeds the 317 foot Vernal Fall, "is still running at spring conditions with significant water levels resulting in a swift, dangerous current," it said.

The Mist Trail which leads to Vernal Fall is one of the most popular hikes in the park, with over 1,500 people hiking up it daily.

The feared deaths follow the deaths of one hiker on the Merced River in May, and those of two hikers who drowned in the Hetch Hetchy Reservoir at the end of June.

The Mist Trail was closed Tuesday as a search was launched, but reopened on Wednesday. "Yosemite National Park Rangers will continue search efforts throughout the day," said the park statement.