Thailand's worst floods in decades are a baptism of fire for Prime Minister Yingluck Shinawatra, observers say, as the political novice struggles to get a grip on a disaster that could define her career.

The 44-year-old former businesswoman, tapped by her fugitive brother Thaksin Shinawatra to run for office in his place, has been thrust to the forefront of a crisis that has killed hundreds of people and now threatens the capital.

At times the mother-of-one has showed signs of strain, appearing teary-eyed at news conferences and describing the crisis as overwhelming, while her political enemies have sought to use the occasion to undermine her popularity.

Her administration has been accused of initially responding too slowly and giving confusing information, although the strongest criticism has so far been directed at other officials rather than Yingluck herself.

"Yingluck has not lacked leadership," said Paul Chambers, a Thailand expert at the University of Chiang Mai in the north of the country.

"However, given that she is already saddled with the feeling among many that she is a mere proxy with no political experience, it makes it hard for her to suddenly earn her worth in these difficult flood-water times," he told AFP.

Experts say the seeds of the crisis were sown before Yingluck took office, as officials retained too much water in reservoirs, while the premier has had to deal with sometimes intransigent bureaucrats and Bangkok authorities.

The telegenic former president of a Thai real estate firm was swept to power on the back of the vast popularity of her brother Thaksin -- who was ousted in a 2006 coup -- among poor Thais, particularly in rural areas.

Now it is the very people who voted for her who are suffering the most as the government focuses its efforts on protecting the wealthy capital, the traditional heartland of the rival Democrat Party.

At first Yingluck, who has vowed a CEO-style premiership, appeared hesitant about how to deal with the deepening crisis, repeatedly reassuring people in Bangkok that the capital was safe as the waters drew closer.

Nonetheless she earned public admiration by spending her weekends visiting flood-stricken areas and posing for photographs with flood victims during the early days of the crisis.

Not all of the images have been PR coups, however -- one picture of her wearing rubber boots made by the Burberry luxury brand sparked a flurry of Internet chatter, not all of it favourable.

An initial strategy of blocking the floods upcountry failed and, in probably her most difficult decision yet, Yingluck on Thursday ordered the capital's floodgates to be opened to drain water through its canals.

"Yingluck has done her best but it has been insufficient. She has needed to be more decisive at times, but she is learning now," said Thitinan Pongsudhirak, a political scientist at Bangkok's Chulalongkorn University.

"Now you see more coordination than before -- she seems more in charge now," he added. "This crisis is going to define her premiership."

The disaster has already taken a heavy toll on the economy and the vital tourism sector, still recovering from deadly political unrest last year, and Yingluck warned Saturday flooding would last for four to six more weeks.

There have been signs of tensions between the prime minister and Bangkok governor Sukhumbhand Paribatra, a rival Democrat, who has asked voters to ignore information from the central government and listen to him only.

Underscoring her growing frustration with the governor, Yingluck on Friday asserted her authority by invoking a disaster law to take nationwide control of the emergency response.

"In this particular case, my sense is that she acts as a CEO delegating responsibilities to the various ministries and politicians," said Pichai Chuengsuksawadi, editor-in-chief of the Bangkok Post, an English-language daily.

"She's not a politician and politicians know how to manage and have a track record of influence over government officials."