WASHINGTON — The Justice Department said today it has reached an agreement to allow the merger of US Airways and American Airlines.

The agreement requires the airlines to scale back the size of the merger at key airports in Washington and other big cities.

In August, the government sued to block the merger, saying it would restrict competition and drive up prices for consumers on hundreds of routes around the country.

The airlines had said their deal would increase competition by creating another big competitor to United Airlines and Delta Air Lines, which grew through recent mergers.