The Pentagon has asked Congress to limit official travel to Afghanistan during a new fighting season so helicopters and other support would be available for military use rather than to transport lawmakers, a senior military official said Friday.

Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, and Defense Secretary Ashton Carter asked that travel be limited to committee chairmen, ranking members and congressional leadership, said the official, who was not authorized to speak publicly about the request.

The official said it was not unusual to ask members to limit travel so that resources such as helicopters are available for transporting American troops in Afghanistan. Similar requests have been made in the past in advance of a new fighting season.

Fighting in Afghanistan traditionally intensifies in the spring and summer when snows melt and mountain roads are passable.