TOKYO/FRANKFURT (Reuters) - Airlines pulled in extra, larger aircraft on Thursday to help thousands of people leave Tokyo, and some began screening planes, passengers and crew for radiation as Japan raced against nuclear disaster. As an increasing number of governments from Britain to New Zealand to South Korea advised citizens to leave quake-affected northern Japan, airlines mobilized for a stream of mainly outbound traffic from one of the world's biggest cities. Japan has been taking desperate measures to contain a crisis at the Fukushima nuclear power plant, crippled by the earthquake and tsunami that devastated the northeast on Friday. "We can keep flying even if the situation deteriorates further. But never say never; we don't know what will happen there over the coming days and weeks," Christoph Franz, chief executive of Germany's Lufthansa, said in Frankfurt. The US State Department said the government had chartered aircraft to help Americans leave Japan and had approved the voluntary departure of family members of diplomatic staff in Tokyo, Nagoya and Yokohama - about 600 people. The US travel advisory came after Australia urged citizens with nonessential roles in Japan to consider leaving Tokyo and the eight prefectures most damaged by the earthquake. Australia cited infrastructure problems rather than nuclear concerns. "We have a real problem in terms of the infrastructure in Japan. We have uncertainty of power supply, we have problems with train services, we have problems with public transport services, many schools have closed, and there is this repeated series of aftershocks," Australian Foreign Minister Kevin Rudd said. Britain said it was chartering flights from Tokyo to Hong Kong, which would cost 600 pounds per person. Britons directly affected by the tsunami would not be charged. France and Germany have also advised citizens in Tokyo to get out or head to southern Japan. France's ambassador in Tokyo, Philippe Faure, said he expected about 500 nationals to be flown out on two special Air France planes on Thursday, with 700 to 800 leaving the following day. Air India increased flights and used bigger planes to help bring back Indians from Japan. It has been using a Boeing 747-400 jumbo jet with a capacity of 423 passengers, a spokesman said. Meanwhile, authorities worldwide responded to concerns about the possible health impact from radiation by starting checks on people, planes and boats. About 25 passengers arriving in Taiwan from Japan were found to have radiation levels slightly higher than normal, a government official said on Thursday. Authorities in South Korea reported unusually high radiation levels on three passengers arriving from Japan. Malaysia, which has been screening air passengers, bags and aircraft since Monday, said it would scan ports. US Homeland Security Secretary Janet Napolitano said passengers and cargo arriving in the United States from Japan were being checked out of an abundance of caution and that no radiation had been detected.