Their eyes lined with black pencil and lips painted red, women in sequined saris line the labyrinth of squalid lanes that make up Sonagachi, one of Asia's largest red light districts in the old quarters of this bustling eastern Indian city.

In front of open sewers, they chat on mobile phones and flirt with customers, who follow them into the dark doorways of decrepit brothels, up winding staircases into tiny rooms with just a bed, television and posters of Hindu gods on the walls.

In the global battle against HIV/AIDS, sex workers like those in Sonagachi are a crucial link in a chain of infection that some 20,000 experts gathered in Washington are debating how to break -- but without having foreign sex workers there.

U.S. travel restrictions on visas for sex workers mean thousands of them have been unable to attend the annual International AIDS Conference (IAC), the world's largest forum to discuss policy on fighting the deadly virus.