U.S. President Barack Obama paid tribute to the victims in a deadly subway collision in northeastern Washington, D.C. on Monday.

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

The statement was issued hours after one subway running on the Red Line smashed into the other one that stopped on the track, leaving at least six killed and many others injured.

The capital and Maryland authorities have sent a large number of firefighters, policemen and emergency rescuers to the scene, where the Washington mayor, Adrian M. Fenty, monitored the process.

"I want to thank the brave first responders who arrived immediately to save lives," said Obama. "My staff has been in touch with Mayor Fenty's office and will continue to monitor the situation."

The investigation of the cause is still undergoing and the further information was expected to be available on Tuesday morning, said officials.

Source: Xinhua