"We cannot afford to wait until a hazardous volcano begins to erupt before deploying a modern monitoring effort. The consequences put property and people at risk -- including volcano scientists on site and pilots and passengers in the air," USGS Director Chip Groat said in a statement.

The report, published on the Internet at http://pubs.usgs.gov/of/2005/1164/, is the first comprehensive review of the 169 US volcanoes, the USGS said.

It calls for a National Volcano Early Warning System that would be up and running 24 hours a day.

The states of Alaska, California, Washington, Oregon, Hawaii, Wyoming, and the Commonwealth of the Northern Marianas all have have dangerous volcanoes with monitoring gaps or no monitoring in place, the USGS said.

"To help keep communities safe, it is essential to monitor hazardous volcanoes so that we know when unrest begins. This is the only way to forewarn communities at risk in enough time to activate emergency response plans, and ultimately help save lives and property," Groat said.

The USGS says that since 1980, 45 eruptions and 15 cases of "notable volcanic unrest" have occurred at 33 US volcanoes.

Some high-profile volcanoes, such as Mount St. Helens in Washington and Kilauea in Hawaii, are carefully watched.

But there are 13 very high threat volcanoes with inadequate monitoring, including nine in the Cascade Range of the Northwestern United States and four in Alaska.

And there are 19 in Alaska and the Mariana Islands that could damage aircraft that fly over them, the USGS said.

"We nearly lost a fully loaded Boeing 747 to volcanic ash cloud in Alaska in 1989," said Ed Miller of the Air Line Pilots Association.