A new mobile telephone emergency alert system will be launched in New York City and Washington DC by the end of the year, US officials have said.
The system will allow the federal government and local authorities to reach people on their mobile phones to warn them of imminent danger.
Messages from the US president or information about missing children would also be sent through the system.
New York Mayor Michael Bloomberg said there would be no charge to users.
Mr Bloomberg unveiled the Personal Localized Alerting Network, or Plan, on Tuesday, explaining that the system will implemented through a special chip installed on new mobile phones.
Officials anticipate that the ability to receive the notifications will spread as mobile users update their phones.
Speaking at a news conference near Ground Zero - the site of the attacks on New York on 9/11 - Mr Bloomberg was accompanied by Federal Communications Commission Chairman Julius Genachowski.
Mr Genachowski said every mobile phone carrier is expected to participate.
Carriers may allow their clients to opt out of receiving notifications from local officials and information about missing children, but receiving presidential alerts will be mandatory.
The system works through GPS technology and will send some of the alerts based on a user's location.