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People in four US states were left unable to make 911 calls on Wednesday night after widespread system issues.

Many services have now been restored after reports that lines went down for call centres across Nevada, Nebraska, South Dakota and Texas.

Federal officials have previously warned that the move to digital systems by emergency services has brought with it the risk of cyber-attacks.

But a cause of last night's problems has not yet been given.

In Las Vegas, Nevada, issues were reported for several hours before police said services had been restored shortly after 22:00 local time (04:00 GMT).

Officers had earlier said they had been able to see when an emergency call was coming from a mobile device, meaning they could phone the caller back.

But they urged the city's hundreds of thousands of residents against ringing to test the connection.

That was echoed by the Rapid City Police Department in South Dakota, which said shortly before 21:00 local time that local handlers had been "inundated" by callers trying to see if the line worked.

Officials said issues were unfolding "throughout the state", but said the text-message function remained available for people to make emergency contact.

Meanwhile, local forces across Nebraska used their Facebook pages to offer callers alternative numbers to ring - before updating that they, too, were back online.

Police in the Texan city of Del Rio said there had been an issue limited to one cellular provider, insisting that there was no issue with their call systems - nor with the emergency services themselves.

Federal officials are yet to comment on the incident.

The National 911 Program has previously highlighted the benefits of the digitisation of emergency services nationwide, while warning that this comes with "the expanded risk of cyber-security attacks".

One such attack in 2017 took 911 centres offline in more than a dozen states, according to the Reuters news agency.