JOPLIN, Missouri (AP) - Rescuers refused to be deterred from their efforts to find survivors beneath Joplin's jagged piles of tornado rubble, even as the death toll rose Wednesday to 125.

No new survivors had been pulled from the city's wrecked neighborhoods, but determined crews carried on with the search, checking some areas for a fourth time since Sunday's disaster. They planned to do a fifth sweep, too.

``We never give up. We're not going to give up,'' City Manager Mark Rohr told a news conference. ``We'll continue to search as we develop the next phase in the process.''

Rohr raised by three the death toll of the nation's deadliest single tornado in more than 60 years. The estimated number of injured climbed to more than 900.

At least 50 dogs have joined the search, and the teams were also using listening devices in hopes of picking up the faint sound of anyone still alive beneath the collapsed homes and businesses.

``We've had stories from earthquakes and tsunamis and other disasters of people being found two or three weeks later,'' Fire Chief Mitch Randles said. ``And we are hopeful that we'll have a story like that to tell.''

Searchers ``try to get into every space. We're yelling. We've got the dogs sniffing. We've got listening devices,'' Randles said.

Meanwhile, roughly 100 people were reviewing information about individuals who were reported missing after the storm. Rohr said the group was making progress, but he declined to say how many remain unaccounted for.

Authorities have cautioned that people who are unaccounted for are not necessarily dead or trapped in debris. Many, if not most, of them probably survived the storm but have failed to tell friends and family where they are.

The Joplin tornado was the deadliest single twister since the National Weather Service began keeping official records in 1950. It was the eighth-deadliest in U.S. history.

Scientists said the system was an EF-5, the strongest rating assigned to tornadoes, with winds of more than 200 mph (320 kph).

It also appeared to be a rare ``multivortex'' tornado, with two or more small and intense centers of rotation orbiting the larger funnel.

Bill Davis, the lead forecaster on a weather service team sent to survey the damage, said he would need to look at video to confirm that.

But, he said, the strength of the tornado was evident from the many stout buildings that were damaged: St. John's Regional Medical Center, a bank that was destroyed except for its vault, a Pepsi bottling plant and ``numerous well-built residential homes that were basically leveled.''

Davis recalled his first thought on arriving in town to conduct the survey: ``Where do you start?''