American Airlines said today it had lost two planes with a total of 156 people on board.
"American Airlines confirmed today that it lost two aircraft in tragic incidents this morning," the airline said in a statement.
One was Flight 11, a Boeing 767 en route from Boston to Los Angeles with 81 passengers, nine flight attendants and two pilots, it said.
The other was Flight 77, a Boeing 757 operating from Washington Dulles to Los Angeles with 58 passengers, four flight attendants and two pilots.
"We are horrified by these tragic events," said American Airlines chairman and chief executive Mr Donald Carty in the statement.
"Our thoughts and prayers go out to the families of all involved".
The airline said because of the heightened security it was working closely with US government authorities and would not release more information at this time.
The US government had shut down the entire civil air traffic system in the United States, it noted. American, TWA and American Eagle would not operate, it said.
People wanting to know about relatives should call American Airlines response number at 1800-2450999.
AFP