Only a few weeks later another major engagement developed west of the Black River line at Nà Sản, a strong point held by nine full-strength French battalions, supported by aircraft and five batteries of 105-mm. howitzers. Cut off from overland supply, Nà Sản was a fortified area of about fifteen square kilometers with an airstrip on the floor of a valley. The French fortified the surrounding high ground and ran supplies to the beleaguered fortress by air. The Việt Minh attacked Nà Sản with two regiments. The major fighting occurred between November 23 and 30. The Việt Minh attacked tenaciously, but the French defenses held.