Organised crime gangs are using Chinese officials' passports to smuggle mainland immigrants through Kai Tak airport to the United States and Australia, diplomats said yesterday. Corrupt cadres are selling their passports in a trend which has become so widespread that Western nations have begun closely scrutinising People's Republic of China officials travelling abroad. They are suspected of obtaining visas by submitting the passports to embassies with batches of official travel documents that are usually stamped without question, said a source. It is the latest development in a surge of illegal immigration through Hong Kong airport which has alarmed the US, Canada and Australia. 'They hope that when they are travelling with the service passports, authorities will not look closely at them,' a Western diplomat said. The green service passports, which differ from the ordinary brown or red PRC travel documents, are easy to doctor - a razor is used to cut out the official's photograph, replacing it with that of the illegal immigrant. They usually come with letters of invitation which are forged or requested from unsuspecting companies interested in doing business with China. Immigration officers in the US and Australia are now inspecting the letters when 'officials' arrive with them, said the sources. 'A lot of them we only catch upon entry at the airport, when we check out the letters,' a US official said. Groups of illegal immigrants carrying the passports were caught trying to enter the US earlier this year. While smugglers have been using Kai Tak to send their human cargo to the US over the past two years, according to an official, they are now using Macau as a transit point. Australia has become the latest destination for illegal immigrants. Authorities have caught more than 50 mainlanders trying to use the forged documents.