WASHINGTON — The government will soon be pinging cell phones and posting on Web sites to warn Americans of impending disasters as it updates its Cold War-era emergency alert system.

By the end of next year, the Homeland Security Department expects to be able to send emergency alerts to cell phones, Internet sites and hand-held computers to reach as many people as possible before a catastrophe strikes, spokesman Aaron Walker said yesterday. It also seeks to transmit warnings on cable TV channels and satellite radio to supplement the government's long-tested but never-used national alerts on network channels and AM radio stations.

"Anything that can receive a text message will receive the alert," said Walker, a spokesman for the Federal Emergency Management Agency, which is running the alert system for Homeland Security. "We find that the new digital system is more secure, it's faster and it enables us to reach a wide array of citizens and alert them to pending disasters."

In 1951, President Harry Truman created the nation's first alert system, which required radio stations to broadcast only on certain frequencies during emergencies. That evolved into the test on TV and radio stations throughout the Cold War that solemnly intoned: "This is a test of the emergency broadcast system. This is only a test."

Only the president can order a national emergency alert. The system was initially designed to warn Americans of a nuclear attack, but President Bush last month ordered Homeland Security to extend the alert "for situations of war, terrorist attack, natural disaster or other hazards to public safety and well-being."