A widespread 911 outage affected four US states yesterday.

The services have since been restored, but no cause has been given for the outage, which impacted all of South Dakota and parts of Nebraska, Nevada, and Texas.

– Getty Images

The outages meant that some residents couldn't reach the emergency services during the downtime across multiple cities, which lasted for a couple of hours.

Authorities across the four states reported 911 outages yesterday evening, including the Department of Public Safety in South Dakota, police in Las Vegas, the Nebraska county of Douglas, and the Texas city of Del Rio.

Police in Del Rio said the outage had to do with an issue with a major cellular carrier, reported Reuters.

The Department of Homeland Security has previously warned that cyber attacks against 911 services are more likely following the migration to digital systems based on Internet Protocol standards.

Such attacks have occurred in the past, notably in 2017, when 911 centers were impacted in more than a dozen states.

Earlier this year, AT&T was hit by a widespread outage which meant its first emergency services network FirstNet was out of service.

In a statement to DCD, AT&T said that its network saw no issues during the outage yesterday.

"Our network is operating normally. There appeared to be an issue on another carrier’s network that could have affected calls to 9-1-1," said an AT&T spokeswoman.

Australian telco Telstra suffered a similar outage in March, impacting Triple Zero (000) calls, the country's equivalent to the 911 emergency service phone number.