MINNEAPOLIS — Millions of smartphone users have begun receiving text messages about severe weather from a sophisticated government system that can send a blanket warning to mobile devices in the path of a dangerous storm.

The National Weather Service’s new Wireless Emergency Alerts system offers a new way to warn Americans about menacing weather, even if they are nowhere near a television, radio or storm sirens.

Started Thursday, the system notifies people about approaching tornadoes, hurricanes, blizzards and other threats. When a warning is issued for a specific county, a message of no more than 90 characters will cause late-model smartphones in that area to sound a special tone and vibrate.

Users do not have to sign up for the service or pay for the text message. And people who prefer not to get the warnings can opt out of the system.

“These alerts will make sure people are aware of any impending danger and provide them with the information needed so they can be safe until the threat is over,” said Amy Storey, spokeswoman for CTIA-The Wireless Association, an industry trade group that helped set up the system.

The system does not yet work with all smartphones or in all areas. It is part of a broader alert network the Federal Emergency Management Agency launched in April that can also send public-safety warnings from the president and participating state and local governments. But the weather service estimates that more than 90 percent of the messages will be about storms.

The weather warnings will include tornadoes, hurricanes, typhoons, tsunamis, flash floods, extreme winds, blizzards and ice and dust storms.

Designers were concerned about overloading users with too much information, so they deliberately limited the messages to warnings, not watches, and excluded severe thunderstorm warnings, weather service spokeswoman Susan Buchanan said.

Wireless carriers serving almost 97 percent of U.S. subscribers have agreed to participate, including the biggest nationwide companies — AT&T Inc., Verizon Wireless, Sprint Nextel Corp. and T-Mobile USA. Each of the four offers at least some phones capable of receiving emergency alerts, with more on the way.

Sprint, Verizon and T-Mobile say they offer the service nationwide. AT&T only offers it in New York City, Washington, D.C., and Portland, Ore., at the moment. Spokesman Michael Balmoris said the company will add additional markets over time but declined to say which ones or when.

Government officials don’t have a good handle on exactly how many capable devices are already in use, but Damon Penn, assistant administrator for national continuity programs at FEMA, said the number is probably in the millions.

He said smartphone users should check with their carriers to find out whether service is available and if their device is able to use it. He said many people own phones equipped to get the new alerts but don’t know it yet.