Airline tickets sold out and private jet firms were swamped with bookings on Thursday as foreign nationals scrambled to leave Japan after more governments advised their citizens to flee.

With fears escalating over a nuclear crisis triggered by Friday's huge quake and tsunami, Britain and Australia were among the nations advising citizens to leave Tokyo, while the United States chartered special flights.

The well-heeled were taking drastic measures as Japanese engineers battled to prevent a major radiation leak from the stricken Fukushima nuclear plant, located 250 kilometres (155 miles) northeast of the capital.

Iris Riesen, managing director of Jet Aviation Hong Kong, said the Chinese territory's private jet firms had seen a huge jump in requests from people desperate to get out of Japan quickly.

"There has been high, high demand - we've been flooded with requests," she told AFP, adding that a return flight between Hong Kong and Japan costs between $90,000 and $140,000 depending on the size of the plane.

Riesen said the firm only had one 14-seat G550 jet available, while it manages 17 other planes for private owners, some of whom have ordered flights for themselves or friends to get out of Japan.

As many expatriates headed to other Asian capitals, some said they were considering leaving the region altogether.

"I know we're a great distance from the nuclear plant and I don't feel immediately threatened, but I'd rather be safe than sorry," said Jeanette Hoch, a Swedish mother-of-two who moved to Hong Kong last year.

The 37-year-old - who said some of her expatriate friends were considering taking extended holidays abroad - added her husband's company said it would move employees if there was a chance Hong Kong could be at risk.

The jittery mood defied assurances from authorities that there was no risk currently in Tokyo, let alone further afield, even though radiation leaks at the plant itself have briefly reached dangerous levels.

The US State Department chartered flights for Americans wishing to flee Japan and authorised its embassy staff and their families to leave the country, after US nuclear regulators questioned safety measures taken by the Japanese.

But with experts offering contradictory opinions on how serious the situation is at the Fukushima plant, one French businessman with operations in Japan said his company was "struggling to find a consensus".

He said if the company evacuated foreign staff, it would also have to offer passage to worried locals.

"There is a huge perception gap between the Japanese in Tokyo and the group's management in France," the man told AFP on condition of anonymity.

"This is not a company accustomed to working in countries at risk, so we do not know how to cope with such a situation. There is not even a crisis-management team in Paris."

China moved thousands of its citizens to Tokyo for evacuation from the country and France said it was assigning two government planes to pull its people out.

Several large Nordic companies, including IKEA and H&M, offered to help their Japanese employees leave Tokyo and surrounding areas and relocate further south.

Commercial flights were under pressure, with just a handful of seats left on most services from Narita - which serves Tokyo - to Hong Kong, Singapore or Seoul.

Demand was driving the average price of a one-way ticket above $3,000, far higher than the normal price.

Australia's Foreign Minister Kevin Rudd said it was the "breakdown of essential services" that had prompted Canberra's evacuation warning, not merely radiation fears, with schools closed and power and transport badly disrupted.

Some firms and professionals - Japanese and foreign - in Tokyo were rushing their operations to the western hub of Osaka, where hotels quickly became booked out and there was a squeeze on office space.