Authorities in Thailand are turning to desperate measures as they try to prevent the Chao Praya River from overflowing its banks and inundating central Bangkok.

The Thai navy has deployed a fleet of ships to the Chao Praya and other rivers, where they are using their propellers to try to speed the flow of water through the city to the sea.

Navy Captain Prasert Chanprapak said he believes the effort is helping.

He said with 10 vessels in one location, he believes the flow of water has been doubled.

But Prime Minister Yingluck Shinawatra says that, despite the navy efforts and constant reinforcement of the embankments, she is no more than 50 percent certain that the city will be spared.

In an address to the nation Tuesday evening, she said that if the embankments fail, the city could be flooded to a depth of 1.5 meters in places.

If that happens, she said, authorities will use all available means to see that the water drains off as quickly as possible. Officials say the most critical moment will come at 7 a.m. local time Thursday when an unusually high tide coincides with a surge of floodwater from the north.

Ms. Yingluck's warning sparked another wave of panic buying by residents, who have emptied store shelves of supplies. Many are frustrated and confused after a series of sometimes contradictory statements by national and local officials.

Resident Wanthawat Songsirisri said he doubts the authorities can protect the city.

He said he thinks the volume of water descending on the city from central Thailand is just too much for the government to handle.

The government has ordered a five-day holiday beginning Thursday to allow residents to prepare for the worst or leave the city.