SAN DIEGO ? US experts called Friday for toxicity tests on chemicals they suspect play a role in the development of breast cancer, a leading cause of death in American women.

"We're currently not identifying chemicals that could be contributing to the risk of breast cancer," said Megan Schwarzman, a physician and environmental health researcher at the University of California, Berkeley.

According to Schwarzman, only a handful of the more than 200 chemicals in the environment linked to mammary tumors in lab animals have been regulated by the US authorities "on the basis of their ability to cause breast cancer."

She was speaking at a major science gathering, the annual meeting of the American Association for the Advancement of Science in San Diego.

Schwarzman is part of a panel of experts set up last year to identify which chemicals cause breast cancer and to develop toxicity tests to identify them.

The Breast Cancer Chemical Policy Project is expected to submit a report to health authorities in April.

As the incidence of the most common invasive cancer in women has skyrocketed in a generation, a flurry of studies has looked into the role of chemicals in breast cancer.

Treatment and survival rates have improved, but scientists have been running to stand still when it comes to pinpointing what causes breast cancer, said panel member Sarah Janssen, a physician and scientist with the Natural Resources Defense Council.

"Although we've made great strides in improving treatment and breast cancer survival rates, really we don't know much about preventing breast cancer... and most of the causes are not well understood," she said, noting hypotheses that environmental exposure affects breast development and the risk of disease.

"People are exposed to dozens of chemicals in their daily activities and biomonitoring has detected hundreds of chemicals in the fetal cord blood, in breast milk, adult blood and urine."

Only around a quarter of more than 186,000 women diagnosed with breast cancer in 2005 were genetically predisposed to the disease, and other breast cancer risk factors, including the early onset puberty in girls, have been linked to chemicals.