Heavy shelling north of Yemens 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 capital, Sana, 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 heavy fighting in the north of the city. There, large numbers of tribal fighters surging south toward 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 tried to seal off the city to prevent rural tribesmen from joining the fight there.