The New York Times reports:

Heavy shelling north of Yemen’s capital threatened to close the main international airport Thursday as government troops and opposition tribesmen appeared to escalate bloody street battles that have pushed the country to the edge of civil war.

The airport, which lies roughly six miles north of the city, was open on Thursday and flights operated normally, the airport director, Naji Quddam, said in a statement, denying earlier news reports that it had closed.

But the main road to the airport from Sana remained dangerous to navigate because of government checkpoints, sporadic shelling and heaving fighting in the north of the city.

There, large numbers of tribal fighters surging south toward the capital, Sana, squared off against Yemeni troops at an important checkpoint in fierce fighting overnight and on Thursday. The northern checkpoint is a major barrier between the capital and Amran Province, a stronghold of the tribesmen loyal to the Ahmar family who have been battling the government for 10 days. Government troops have attempted to seal off the city to prevent rural tribesmen from joining the fight there.

On Wednesday afternoon, tanks and armored vehicles could be seen rolling into Sana from the south. The streets of city were largely empty, as residents fled for the safety of surrounding villages. Exploding artillery shells and sporadic machine-gun fire could be heard across the city.

Despite his repeated public offers to step aside to ease the crisis in the country, Yemen’s authoritarian president, Ali Abdullah Saleh, appeared to be gearing up for a major assault on the Ahmar family, his tribal rivals and political opponents.

The violence here has transformed a largely peaceful uprising into a tribal conflict with no clear end in sight. The United States and Yemen’s Arab neighbors like Saudi Arabia, which have tried and failed to mediate a peaceful solution to the country’s political crisis, are reduced to sitting on the sidelines and pleading for restraint.