South Vietnamese troops formed a new line North of Hue yesterday in an attempt to defend the city against a general North Vietnamese offensive.

Thousands of civilians are leaving Kontum, the central highlands provincial capital, as North Vietnamese troops move steadily south along Highway 14. U.S. and South Vietnamese officials expect an attack on the city, but no one is certain when it will come.

The North Vietnamese are moving south toward Hue after their victory at Quang Tri. A U.S. officer said an attack on Hue, 32 miles south of Quang Tri. could come at any time. He added that South Vietnamese commanders are confident that their forces can successfully defend Hue.

A North Vietnamese broadcast said that Thua Phien Province, of which Hue is the capital, is doomed.

The South Vietnamese army is depending on U.S. air power to play a major part in defending Hue. For the first time in the war, five U.S. aircraft carriers are operating off the coast.

Brigadier General Thomas W. Bowen said that a major problem in defending Hue is that North Vietnamese troops and agents will probably infiltrate toward Hue by mingling with refugees pouring south from Quang Tri.

An escape route to Pleiku from Kontum was opened to civilian traffic yesterday for the first time in ten days. It had been blocked by a North Vietnamese regiment.

Civilians, looking for ways to escape Kontum before the North Vietnamese attack, have gathered at U.S. helicopter landing pads.

Some military authorities believe that the North Vietnamese will not attack Kontum until there is had weather. Bad weather would blunt U.S. air power. Others say that the North Vietnamese are steadily consolidating and regrouping their forces and will attack when this move is complete.