(Reuters) - U.S. drug regulators found "significant violations" of manufacturing regulations at Mylan Inc's Caguas, Puerto Rico, facility, according to a letter posted on Tuesday.

A Food and Drug Administration inspection earlier this year found that the generic drugmaker failed to complete a required test meant to ensure uniformity of the medicine content, regulators said in a warning letter dated October 13.

Mylan also did not thoroughly investigate or explain why a batch of medication or its components did not meet specifications, FDA said.

A spokeswoman for Pittsburgh-based Mylan, the world's third-biggest generic drugmaker, said the company has provided its written response to the FDA and plans to work with the agency to address the issues outlined in the letter.

"(We) believe we will be able to resolve this matter expeditiously," spokeswoman Nina Devlin said in an email. "Production and shipment of product from the Caguas facility continues uninterrupted."

In the letter, FDA asked the company to tell the agency within 15 days of receiving it how the drugmaker was correcting the discovered violations or explain why the corrections were delayed.

If Mylan failed to improve, it could face legal action and hurdles receiving contracts from federal agencies or approval of its products.

Mylan shares were up 1.5 percent at $17.92 in morning trading on the Nasdaq.

The FDA posted the letter at: http://r.reuters.com/vyr25s.

(Reporting by Alina Selyukh in Washington, editing by Gerald E. McCormick)