Nov 7 (Reuters) - A U.S. judge on Thursday gave Southwest Airlines permission to file a brief asking that US Airways Group and American Airlines be required to give up take-off and landing slots at key U.S. airports should they merge.

Southwest has expressed interest in acquiring rights at New York's LaGuardia and Reagan National Airport near Washington that could become available if the merger goes ahead.

In a statement, the Dallas-based carrier said its filing would explain the benefits it could offer consumers should it be allowed to expand at those airports. The judge hearing the case said the brief must be filed no later than Nov. 15.

The Justice Department filed a suit in August to block the merger between American Airlines. U.S. Airways and Attorney General Eric Holder said this week the government wanted the carriers to shed slots at Reagan National and other "key" U.S. airports.

Holder said he hoped to reach an agreement settling the matter before a federal trial is due to start Nov. 25.

American and US Airways declined to comment on Thursday.

US Airways told its shareholders in July that Southwest and JetBlue Airways were pushing hard to influence regulators to require it and American to shed slots in the merger that would form the world's biggest carrier. .

JetBlue told the Reuters Aerospace and Defense Summit in September that a merged American and US Airways should not have a share of slots at Reagan National that exceeds US Airways' current standalone share, which is about 55 percent .

Other U.S. carriers have cited potential benefits from the merger. Allegiant Travel, a Las Vegas-based low-cost carrier that serves leisure destinations, said on Thursday it would consider opportunities to acquire airport slots that might open up as a result of the merger.

"We would certainly consider any and all opportunities, however, ultimately economics are a significant consideration for us in any market," Allegiant spokeswoman Jessica Wheeler said in an email.

"We anticipate that the merger would create a healthy network carrier that would focus on the needs of business and international travelers, opening up growth opportunities in U.S. leisure markets for leisure-focused carriers such as Allegiant," Wheeler said.