WASHINGTON - A rush-hour subway train plowed into the back of a stalled train in its path on Monday in the northern reaches of the U.S. capital, killing at least six people and injuring 70.

The female operator of the trailing train was among the six killed.

The train in front of her was stopped on the tracks on one of the city's heavily travelled subway routes, apparently awaiting instructions to proceed.

Rescue crews used heavy equipment to cut into the wreckage, with the first car of one six-car train wedged underneath the other after rear-ending it at high speed.

There was a gaping hole in one train, with the orange seats from the subway scattered along the tracks.

The 5 p.m. EDT accident shattered the calm in a quiet residential neighbourhood near the D.C.-Maryland border where many backyards overlook the line.

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

"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."

President Barack Obama sent his condolences to the victims of the crash.

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

The president also thanked rescue personnel who helped to save lives.

Mayor Adrian Fenty held a news conference at the scene.

"What we do know is that there are scores of people who have been injured. There are already four confirmed fatalities," he said.

"It is my present understanding that this would then be the deadliest accident in the history of our Metro train transit system."

Alan Etter, a spokesman for the Washington Fire and Emergency Medical Services Department, said the incident was "developing into a mass casualty event."

In addition to the six dead, two more had life-threatening injuries and 12 had moderate injuries. Fifty were classified as being "walking wounded" by authorities.

There were unsubstantiated reports that some passengers were ejected from the trains at the point of impact.

The crash took place between the Fort Totten and Takoma Park stations on a Metro route known as "the red line" that shuttles tens of thousands of commuters every day between D.C.'s leafy Maryland suburbs and the city.

There is a long stretch of occasionally curving track between the two stations, meaning trains often reach higher speeds than they do between stations situated closer together. It appeared as though the trailing train was coming around a bend when the accident occurred.

Several people were taken by gurneys from the wreckage to nearby hospitals as dozens of ambulances transported passengers to nearby hospitals.

Some people remained trapped in the trains after the doors jammed on impact, and rescuers worked to free them hours after the accident - and to search for bodies. Search-and-rescue dogs were also in the cars looking for victims.

Some passengers said they'd received alerts about delays on the line due to work being performed on the track prior to the crash.

The National Transportation Safety Board took charge of the investigation and were searching for the trains' devices that record speed and commands, said NTSB spokeswoman Debbie Hersman.

She said each car was capable of holding 1,200 people, but were less likely to be filled because they were heading downtown during afternoon rush hour.

D.C. police and the FBI also had investigators at the scene to help search the wreckage for any overlooked injured or dead passengers and evidence.

The last major commuter crash in the United States was in September, when 25 people were killed when the conductor of a train in Los Angeles was sending text messages on his mobile phone while driving the train.

The deadly collision Friday in Chatsworth, north of Los Angeles, also injured 134 people and was the worst train accident in the United States in some 15 years.