The city put up numerous posters for the second General Assembly and Conference of the Alliance for Healthy Cities, which was held from Saturday to Monday, Kuaibao said.

More than 400 mayors, health experts and scholars from cities of 21 member countries and regions attended the conference.

On one-third of the poster is a picture of a "nurse" in a pink uniform, holding a large syringe in her hands. It has attracted many people's attention. Some passers-by even said they might have seen the "nurse" before.

Soon after, a netizen pointed out that the woman was Japanese porn star Aoi Minori. In the original full-sized picture of Minori, it shows her legs with her panties half taken-off. The lower part of the picture was cut in the poster.

"We've heard people talk about the poster and took down most of the copies," said the director of Suzhou Health Administration, who was identified as Fu. "We hired an advertisement agency for the promotion of the event and it provided the picture. Maybe workers of the agency downloaded it from somewhere, but we don't know much about the woman on the poster," he added.

Hui Haiming, a sociology professor in Beijing University, said authorities should enhance inspections on the operation of advertisement agencies to prevent them from using pictures off the Internet without copyrights.

A Suzhou lawyer, Zhang Haiyan, said if the actress or her company didn't authorize the Suzhou health authority and the advertisement agency to use the picture, they can sue on the grounds of image infringement.

Some local nurses said they couldn't understand why the organizer of the conference used the picture, as many Chinese nurse images are available, they said.