HOUSTON (Reuters) - Cancer-causing dioxins leached from a Superfund site along a Texas river during Hurricane Harvey flooding, the U.S. Environmental Protection Agency said, triggering calls on Friday for the toxic waste to be permanently moved.

The EPA ordered International Paper and Industrial Maintenance Corp, a subsidiary of Waste Management Inc, on Thursday to conduct testing to determine the extent of the leaching.

The EPA said in a statement that divers found dioxins in a sediment sample in excess of safe levels at one of 14 sites tested. Repairs were made to the damaged cap over the paper mill waste, it said.

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The discovery is "frustrating and frightening," said Richard Mithoff, a Houston attorney representing some 600 residents and businesses who in 2016 sued International Paper and Waste Management, alleging their failure to clean the site exposed residents to the cancer-causing wastes. The lawsuit seeks $400 million in damages. "Some of these people can't just pick up and move," said Mithoff.

International Paper disputed any release of dioxins, saying the rock cap remains "fully intact." It is conducting additional testing "to confirm there was no release around a small area where waste was accessible," the company said in a statement.

A Waste Management spokesman was unavailable for immediate comment.

The former waste dump was declared a Superfund site in 2008 and a temporary rock cap placed over the waste pits three years later. The EPA has proposed removing about 150,000 cubic yards of waste deposited at the site, which sits along the San Jacinto River.

A sample taken by the EPA showed dioxin levels at one site exposed by the storm at 70,000 nanograms per kilogram, more than 2,000 times the legal limit of 30 nanograms per kilogram, according to the agency. The EPA did not respond to requests for comment.

The companies responsible for the site conducted similar repairs to the cap in December 2015, according to the EPA.

Residents claiming damages from the waste dump are to present evidence on Monday before a Harris County District Court judge, along with the EPA's latest discovery of high levels of dioxins, according to Mithoff.

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"We have ongoing discovery now and we are producing medical records and gathering the evidence that we'll need for trial," he said.

(Reporting by Bryan Sims; editing by Gary McWilliams)