Floodwater has pressed toward Bangkok on several fronts as a new area near the international airport began to look vulnerable.
Much of the effort in recent days has been aimed at shoring up defences 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.
Civilians joined soldiers in desperate dike-building efforts after Bangkok's governor delivered a dramatic late-night TV warning that the city had to lay down a million sandbags to protect an especially vulnerable stretch.
North of the city of nine 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.
Outside the capital, the military is helping deliver relief supplies to displaced and stranded residents struggling to survive in half-submerged towns.
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.
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 in.