``The criteria is whether they are going to an area which is highly sensitive, dealing with weapons of war or atomic energy,'' Bruce McKenzie, US consular official in charge of the visa section, told newspersons on Thursday. The US Consulate in Mumbai, the largest consulate in South Asia, has denied visas to `three or four' Indian scientists in the last two months as part of the sanctions imposed by that country soon after India's atomic tests at Pokhran. It does not take decisions on rejecting visas but refers them to the State Department, McKenzie said. Dr R Chidambaram, Chairman of the Atomic Energy Commission became the first scientist to be refused a US visa. However, McKenzie advised Indian scientists to apply well in advance so that the consulate has enough time to process the request.