At least six people were killed and more than 70 others injured during a deadly, rush-hour crash between two commuter trains on a heavily-travelled corridor in Washington.

Some reports put the wounded at 100 as crews scoured the scene to ensure that all of the passengers had been removed from the crash site.

By Monday evening, rescue crews had cut apart mangled train cars and pulled injured passengers from the wreckage.

The crash occurred at around 5 p.m. local time on the Washington Metro system's Red line, not far from the state border with Maryland.

District of Columbia fire spokesperson Alan Etter said the crash is a "mass casualty event," and officials said dozens were evacuated from the site.

Etter told CTV News Channel that crews have been working to ensure that the crash scene is "stable" enough so investigators can get on site and begin assessing the damage.

The Canadian Press reported that one of the trains was stalled on the track waiting for instructions to proceed when it was struck from behind by a second commuter train.

While Etter wouldn't confirm that report, he said that the impact between the two trains was so strong that cars were piled on top of one another.

"Tragically, we have a number of fatalities and injuries as well," he said, noting that the rescue operation had wrapped up by about 8 p.m. local time.

"All the people in need of rescuing have been extricated from the trains," Etter said, adding: "We've never seen anything like this in the city."

In a statement issued Monday night, U.S. President Barack Obama sent his condolences to the victims.

"Michelle and I were saddened by the terrible accident in Northeast Washington D.C. today," he said. "Our thoughts and prayers go out to the families and friends affected by this tragedy."

At the crash scene, one of the train cars was ripped apart and orange seats from its interior were scattered near the tracks.

"It happened so fast, I flew out of the seat and hit my head," passenger Jodie Wickett said.

"The people that were hurt, the ones that could speak, were calling back as we called out to them. Lots of people were upset and crying."

According to Washington Metro spokeswoman Cathy Asato, the fatalities include the female operator who was driving the rear train.

"There have been several injuries and we are still gathering information on the extent of the injuries," she said.

Among the injuries, two were described as life-threatening and 12 others were considered serious. About 50 passengers were described as "walking wounded" by officials.

Despite the crash, service was still running on the system, which carries about 800,000 passengers each day, said Asato.

"We still don't know the cause of the collision. We will be conducting a thorough investigation, but we are not likely to know the exact cause for several days."

The crash occurred between the Fort Totten and Takoma Park stations -- a long, curving stretch of track where trains often pick up speed as they shuttle thousands from the suburbs into the nation's capital.

The system's last crash was in 2004, Asato said. There has not been a Washington Metro collision fatality since January 1982.

However, a transportation expert said the city's infrastructure needs to be examined carefully.

"This is an aging system and one that needs to be looked at very closely," said Peter Goelz, a former managing director with the U.S. National Transportation Safety Board.

In September, 25 people were killed and 134 injured near Los Angeles during a commuter train crash.