TRENTON, N.J. (AP) - The Food and Drug Administration has ordered drugmaker Merck & Co. to correct manufacturing deficiencies at its main vaccine plant.

The agency on Wednesday released a warning letter sent to Merck's chief executive, Richard T. Clark, that states FDA inspectors determined manufacturing rules are not being followed at the plant in West Point, Pa., just outside Philadelphia, which makes a number of children's vaccines.

The nine-page letter refers to what the FDA calls "significant objectionable conditions" found during repeated inspections at the plant between November and January.

Merck officials didn't thoroughly investigate when vaccine batches inexplicably failed to meet specifications, even if batches had been distributed, according to the heavily redacted letter.

The FDA also said that some drug components were tainted, and that the plant didn't have written procedures, tests or other laboratory controls to ensure "strength, quality, and purity" of products.

Merck shares, already hammered this week by FDA rejecting two of its experimental drugs in the past week, initially dropped about $1 on the news, then recovered. They rose 82 cents, or 2.2 percent, to $37.98 in midday trading.

"We have reviewed the warning letter and are committed to addressing all of the FDA's concerns in a timely and comprehensive manner," Merck spokeswoman Mary Elizabeth Blake said in a statement.

The warning letter gives Merck 15 days to tell the FDA how it will correct the violations.

If Merck does not comply, the letter states the FDA can take actions including suspending the plant's manufacturing license and seizing products.

An FDA spokeswoman did not immediately return a call seeking comment.

The plant makes children's vaccines, including liquid PedvaxHIB, to prevent Haemophilus influenza type B, and ProQuad, which protects against measles, mumps, rubella and chickenpox. Is also makes hepatitis A, hepatitis B and meningitis vaccines for children and adults, and Gardasil, to protect young women against cervical cancer.

"Nothing is more important to Merck than the quality and safety of our medicines and vaccines," Blake said.

Last year, the plant recalled 1.2 million doses of its Haemophilus influenza B vaccine because of a sterility problem.