"Today I will frankly tell you the truth. I have left no stone unturned in this crisis but I cannot solve it alone. I need cooperation from all sides," a teary-eyed PM Yingluck told reporters.
"Let's set aside politics. We must work to restore people's morale," she added, in what appeared to be an appeal to the opposition Democrats.
Authorities in the low-lying capital have been racing to reinforce barriers with sandbags in an attempt to protect the city of 12 million people from floods that have killed more than 300 people around the kingdom.
Bangkok governor Sukhumbhand Paribatra -- of the rival Democrat Party -- on Wednesday warned that seven districts in northern and eastern Bangkok were at risk of inundation because of a broken dyke.
He advised residents in those areas to unplug electrical appliances, move belongings to higher ground and study the city's evacuation plan, saying they had 24 hours to prepare for possible flooding.
The new warning contrasted sharply with earlier remarks from the central government suggesting that the threat to Bangkok had eased.
Three months of heavy monsoon rains have damaged the homes and livelihoods of millions of people, mostly in northern and central Thailand, and have forced tens of thousands to seek refuge in shelters.
Currently about one third of Thailand's provinces are affected by the floods, which are several metres deep in places.