MUMBAI (Reuters) - India's largest drugmaker Sun Pharmaceutical Industries Ltd has been warned by U.S. health regulators for violating manufacturing standards at its Halol plant in India, even as it has been working on fixing issues at the plant for over a year.

The U.S. Food and Drug Administration's "warning letter" to Sun Pharma indicates the agency is dissatisfied with the remedial measures the company has implemented since last September, when the FDA first notified the company of its concerns after an inspection.

That inspection report highlighted nearly two dozen issues the FDA staff found, including problems with aseptic practices and water leaks in the ceiling of the manufacturing area.

The letter, which was publicized by Sun Pharma itself and has yet to appear on the FDA website, means the U.S. agency could ban imports from the plant if the problems are not resolved.

The Halol plant makes up about 15 percent of Sun Pharma's sales in its largest market, the United States. It is critical to the company as it has the capabilities to manufacture injectable products, which are difficult to make and hence a niche and lucrative market for drugmakers.

Sun Pharma said in a statement on Saturday that it had made "significant investments" in automation and training to improve quality systems at the plant, hired consultants, and was updating the FDA on all the corrective measures it was taking.

Most of the issues in the warning letter are the same as those in the report sent in September, Sun Pharma's Managing Director and Indian billionaire Dilip Shanghvi, said in a conference call late on Saturday.

The company does not expect any change in the timeline to finish its remediation work and invite the FDA for a re-inspection, he said. The company has not disclosed the timeline.

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Shanghvi said the warning letter may have been issued because Sun Pharma had not "effectively" communicated to the FDA the extent of remediation work it had already done.

"Whether the remediation commitments we made are adequate, whether we need to do more ... these are issues we need to assess and understand and probably consult with additional subject matter experts," he said.

He said the company was also putting together an internal task force to assess how to respond to the issues raised.

The warning to Sun Pharma is the latest in a series of FDA actions over the past year against drug manufacturing plants in India, which supply about 40 percent of the medicines sold in the United States.

Facilities of multinational firms manufacturing in India have also faced similar U.S. action.

Just last month, India's second-largest drugmaker Dr Reddy's Laboratories Ltd received an FDA warning letter citing quality control issues, including data integrity, at three of its manufacturing plants in India.

Sun Pharma itself is already grappling with U.S. import bans on five of its other manufacturing facilities in India.

The company said it was cooperating with the FDA and willing to undertake any additional steps necessary.

Since last September, Sun Pharma has not received any U.S. approvals to launch new drugs made at Halol. The situation will continue until the problems are fixed, Shanghvi said.

"Getting the site back in compliance is a major priority and focus," he said.

The company did not say when exactly the warning letter was issued. The letters are usually posted on the agency's website a week after they are sent to the manufacturer.

(Additional reporting by Neha Dasgupta; Editing by Alison Williams)