Problems with 911 calls in a Texas city along the US border with Mexico were unrelated, officials said, but the widespread outage created concerns about what was causing the problems.

For most agencies, it turned out to be the result of simple human error.

In Kansas City, Missouri, workers installing a light pole for another company on Wednesday cut into a Lumen Technologies fibre line, Lumen global issues director Mark Molzen said in an email to the Associated Press.

Service was restored within two and a half hours, he said.

There were no reports of 911 outages in Kansas City.

Meanwhile, the difficulties some mobile phone callers experienced making 911 calls in in Del Rio, Texas, were apparently because of an outage involving a mobile phone carrier, not the city's 911 system, city spokesman Peter Ojeda said.

Lumen is not a 911 service provider for Texas.

Federal officials were looking into the outage.

"When you call 911 in an emergency, it is vital that call goes through. The FCC has already begun investigating the 911 multi-state outages that occurred last night to get to the bottom of the cause and impact," Federal Communications Commission chairwoman Jessica Rosenworcel said in a statement.

The National Highway Traffic Safety Administration, which houses the National 911 Program, said in a statement that its Office of Emergency Medical Services "is monitoring this issue".

The outages created confusion for some people trying to reach emergency agencies.

The Dundy County Sheriff's Office in Nebraska warned in a social media post on Wednesday night that 911 callers would receive a busy signal and urged people to instead call the administrative phone line.

About three hours later, officials said mobile and landline 911 services had been restored.

Cut fibre lines and other problems have caused 911 outages in recent years in Nebraska.

The issue was worrisome enough that the Nebraska Public Service Commission hosted a hearing on the topic in December.

The South Dakota Department of Public Safety said in statement posted on social media Wednesday night that the 911 service interruption occurred throughout the state.

The agency noted that texting to 911 was working in most locations and people could still reach local law enforcement through non-emergency lines.

Less than two hours later, the agency said service was restored.

Officials in Sioux Falls, South Dakota, said during a news conference Thursday that the outage was unprecedented.

"To our knowledge, we have never experienced an outage of this magnitude or duration," Assistant Fire Chief Mike Gramlick said.

Roger Entner, an analyst with Recon Analytics, said the depth of buried fibre lines varies greatly.

In some places - often in neighbourhoods - the line may be just a few centimetres deep.

Elsewhere, it might be buried much deeper.

Entner said he was surprised by the outage because it is common to have "redundant lines," meaning that if one line is damaged, a backup line will continue to carry the service.

Australian Associated Press