With a flood crisis rocking his sister's government, fugitive ex-Thai premier Thaksin Shinawatra is lying low as prospects of a triumphant homecoming look ever more distant, observers say.

The usually outspoken Thaksin, who lives abroad to avoid a jail term for corruption, has mostly shied away from publicly commenting on a disaster that has killed more than 500 people and threatens the heart of Bangkok.

The former tycoon, 62, has posted several messages on Twitter and Facebook offering sympathy to the victims.

Some relief supplies reportedly also had "with love and great concern from Thaksin Shinawatra" written on them -- without his knowledge, his aides say.

But the one-time policeman has largely remained silent during the disaster and has done little to rally support for his under-pressure sibling, Prime Minister Yingluck Shinawatra.

"Thaksin has generally remained more silent than expected," said Paul Chambers, director of research at the Southeast Asian Institute of Global Studies at Payap University in Chiang Mai.

Thaksin -- who was ousted in a 2006 coup -- might already be distancing himself from Yingluck who could be at risk of losing her job, he said, as she struggles to get a grip on the country's worst floods in half a century.

"Better for Thaksin and other Puea Thai politicians to let Yingluck ride out the storm and take any unpopularity relating to her handling of this crisis with her," said Chambers.

The 44-year-old former businesswoman, whose Puea Thai party won a resounding victory in a July election, has been accused by her critics of acting too slowly and lacking leadership in her management of the crisis.

Her administration has also come under fire for giving contradictory statements about the threat from the rising waters, further rattling nerves in the kingdom.

Thaksin's overthrow heralded five years of political unrest, culminating in street protests by his "Red Shirt" supporters last year, in which more than 90 people died in clashes with the army.

He is despised by the Bangkok-based elite but remains a much-loved figure among many poor Thais.

Other observers said Thaksin -- who once described his sister as his "clone" -- was taking a back seat to give Yingluck a chance to step out of his shadow.

"It's good for him to allow Yingluck to deal with it herself, even though she is failing," said Pavin Chachavalpongpun, a Thailand expert at the Institute for Southeast Asian Studies in Singapore.

He said that any meddling by Thaksin would only "further complicate" the situation, and "open the door for the opposition to attack the government".

Besides, he said, Yingluck's big brother may not be in any position to help. "Thaksin doesn't have a PhD in water management," he quipped.

Yingluck has pleaded for the public's understanding, saying Thailand's dams were already full when she took office in early August, since when the country has been battered by a series of storms.

Thitinan Pongsudhirak, a political scientist at Bangkok's Chulalongkorn University, said he suspected the siblings were "communicating very closely" but there was only so much Dubai-based Thaksin could do from exile.

"The nature of this crisis precludes a more active role -- it's an hour-by-hour crisis," he added.

Yingluck stormed to power on the back of the vast popularity of her brother among poor Thais, particularly in rural areas, many of which have been hit hard by the floods.

In the early days of her premiership, Thaksin appeared keen to boost his profile with controversial trips to Japan and Cambodia, which raised questions about his influence over the government.

He has also made no secret of the fact that he hopes to attend his daughter's wedding in Thailand in December.

But Yingluck has yet to take any legal action clearing the path for his return, such as an amnesty, and according to commentators it is unlikely she will do anything so controversial while the kingdom is mired in crisis.

"Yingluck has been overwhelmed by the floods," said Pavin. "Should Thaksin put another burden on her... I'm not sure she would survive as prime minister. It would be political suicide for both Yingluck and Thaksin."

He said there was "almost zero chance" Thaksin would be back by December, when the country would still be dealing with the aftermath of the floods.