(AFP) - Almost 200,000 people were under evacuation orders in northern California on Monday after a threat of catastrophic failure at the tallest dam in the United States.

Officials said the danger had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, had eased. But people were still being told to stay away.

Several weeks of heavy rain filled the 770-foot (235-meter) high dam to capacity.

The threat comes not from the dam itself, which the California Department of Water Resources said was not in danger of collapse, but an emergency spillway that channels excess water.

A giant hole opened in the dam's main spillway last week, forcing the authorities to activate the emergency overflow channel on Saturday for the first time.

But it began eroding, threatening a rupture that would have sent water surging toward the valley below, media reported.

the authorities released 100,000 cubic feet of water per second from the main spillway, bringing down the level of the reservoir Sunday, the Sacramento Bee newspaper said, quoting the water department.

The paper reported that advocacy groups had warned in 2005 that the spillway posed a danger in the event of major flooding and had recommended to the federal government that it be reinforced.

Department head Bill Croyle told a news conference near the danger zone that he was unaware of the advice, but pledged that engineers would analyze what went wrong once the crisis was over, the paper said.

The department did not respond to a request for comment on the report.

Helicopters readied overnight to drop rocks into eroded areas in the emergency spillway ahead of rain forecast for Wednesday and Thursday that could fill the reservoir again.

The California National Guard said on Facebook that it had alerted its 23,000 members to be ready to deploy.

- Military on standby -

Butte County Sheriff Kory Honea told a news conference on Sunday that no more water was seeping over the spillway, adding: "We're not at the point yet where we can make decisions about whether or not it is safe to repopulate areas."

Pentagon spokesman Jeff Davis told reporters on Monday the federal military was ready, if needed, to provide air transport, water rescue, medical care and shelter.

Some 188,000 people in downstream communities were told to leave on Sunday afternoon as water was still gushing over the top of the wide auxiliary spillway.

Officials in Butte County and Yuba issued "immediate evacuation" orders of low-lying area, with the Butte County Sheriff's department warning of a developing "hazardous situation."

"All Yuba County on the valley floor," the Yuba County Office of Emergency Services said in a Facebook post.

"The auxiliary spillway is close to failing... Take only routes to the east, south, or west. DO NOT TRAVEL NORTH TOWARD OROVILLE!!!!!"

Water from the dam flows down the Feather River that runs through Oroville, a city of about 20,000 people.

"It's clear the circumstances are complex and rapidly changing. The state is directing all necessary personnel and resources to deal with this very serious situation," Governor Jerry Brown said in a statement.

The Oroville Dam has been in use since 1968. Less famous than America's iconic Hoover Dam near Las Vegas, Oroville is still the tallest.

© Agence France-Presse