WASHINGTON (Reuters) - U.S. regulators have warned Merck & Co Inc (MRK.N: Quote, Profile, Research) about manufacturing deficiencies related to vaccine production at a company plant in West Point, Pennsylvania, according to a letter released on Wednesday.

The Food and Drug Administration said it did not believe the manufacturing issues would affect the safety of the company's vaccines. Merck said the warning letter will not hamper its ability to sell currently marketed vaccines.

FDA inspectors found "significant objectionable conditions" during an inspection that began in November 2007 and ended in January, a warning letter from the FDA said.

The FDA letter said the plant makes Merck's Gardasil, a vaccine to prevent cervical cancer that is one of the company's most important and best-selling products, and other products that include hepatitis A vaccine Vaqta and hepatitis B vaccines Comvax and Recombivax.

The letter, dated April 28 and sent to Merck Chief Executive Richard Clark, involved 12 remaining quality-control observations at the facility, Merck spokeswoman Kelley Dougherty said. The letter reflects progress on Merck's part, she said, because an earlier report from the FDA had cited 49 observations.

"This letter does not affect our ability to continue to sell our currently available vaccines," said Dougherty, who noted that the company was continuing its discussions with the FDA on the remaining issues.

Asked if the FDA's ongoing review of possible quality-control lapses could hamper sales of the vaccines over the longer term, Dougherty said she could not speculate.

Shares of Merck were up 71 cents, or 1.9 percent, to $37.85 in midday trading on the New York Stock Exchange amid moderate gains for the drug sector.

(Reporting by Lisa Richwine and Ransdell Pierson, editing by Mark Porter and Gerald E. McCormick)