BANGKOK >> Floodwaters pressed toward Bangkok on several fronts Tuesday as soldiers and residents raced to pile sandbags and officials sounded a new alarm about vulnerable areas east of the capital near the international airport.

Much of the efforts in recent days have been aimed at shoring up defenses along Bangkok’s northern perimeter, facing the brunt of runoff from inland areas where Thailand’s worst flooding in a half-century has killed 315 people. Officials added concerns Tuesday about a new front: a flood plain near Suvarnabhumi Airport.

Droves of civilians joined soldiers in desperate dike-building efforts after Bangkok’s governor delivered a dramatic late-night TV warning that the city had until late Wednesday to lay down 1 million sandbags to protect an especially vulnerable 3.7-mile stretch.

"When the governor said we had 48 hours left, I thought we could not just wait until the water arrived, so I took a day off and volunteered," said Suriya Termchoksap, 39, who took the day off from his job at IBM to help build a dike along a key canal.

North of the city of 9 million people, the government has meanwhile been fighting a losing battle against floods in communities where residents have been trapped on the upper floors of their homes.

Monsoon downpours that began in July have affected two-thirds of the country, bringing life to a standstill in several towns and cities where some areas remain under more than 6 feet of water that is unlikely to dissipate for weeks.

Hundreds of factories have been swamped, and economic analysts say the floods already have reduced projections for economic growth in 2011 to 2.5 percent, down from 4.4 percent, and could inflict about $6 billion in damage — an amount that could double if floods swamp Bangkok.

Outside the capital, the military is helping deliver relief supplies to displaced and stranded residents struggling to survive in half-submerged towns.

"When the northern provinces were flooded, I felt sooner or later Bangkok would be hit," said Santi Pongwinyan, 37, a garment exporter in Bangkok who shut his business Tuesday to join the chains of people building sandbag walls.

"We hope it will be in time," he said.

The Royal Irrigation Department on Tuesday advised residents in districts east of Bangkok near Suvarnabhumi to secure their homes with sandbags and move valuables to upper floors because water levels there were rapidly rising.

A department spokesman stressed that the airport itself was not threatened because it was well-protected.

Bangkok’s other airport at Don Muang, north of Bangkok, is in another area where floods threaten but is also believed to be safe for now. However, the Thai air force, which maintains a base there, said it has moved about 20 planes to other bases as a precaution.

About 10 aircraft have stayed to carry out flood relief missions, but they also may need to be moved if the situation worsens, Air Force spokesman Air Vice Marshal Montol Suchukorn said.

Soldiers were meanwhile hustling to evacuate people from the country’s oldest industrial zone, also north of Bangkok, where about 200 factories were fighting to save their businesses after water starting breaking into the facility two days earlier.

Military trucks took people and their belongings out of buildings and shops inundated with water at much as one yard deep.

Although the government ordered an evacuation of the Nava Nakorn industrial estate on Monday, many factory workers stayed put to try to reinforce their companies’ defenses, with water pumps running nonstop to drain water.