SANAA, Yemen – Thousands of tribesmen threatened Thursday to descend on Yemen’s capital to join the battle against forces loyal to President Ali Abdullah Saleh as the country slid deeper into an all-out fight for power. Government forces in Sanaa unleashed some of the heaviest shelling yet against their tribal rivals in a dramatic escalation of the conflict.

For months, youth-led protesters have tried to drive out Saleh peacefully. But their campaign has been overtaken and transformed into an armed showdown between Yemen’s two most powerful families, the president’s and the al-Ahmar clan. The al-Ahmar family heads the country’s strongest tribal confederation, which has vowed to topple Saleh after 33 years in power.

Their nearly two week-old battle in Sanaa raises a dangerous new potential in Yemen: that tribal fighting could metastasize and spread across the impoverished nation. Tribes hold deep loyalty among Yemen’s 25 million people, and the death of a member can easily draw relatives into a spiral of violence.