PORTLAND Ore. (Reuters) - The U.S. Department of Agriculture has warned a major research hospital that it provides inadequate veterinary care to monkeys at its primate research center, Oregon Health & Science University (OHSU) said on Thursday.

The Aug. 6 warning letter, the lowest-level penalty issued by the USDA, followed an inspection that found that more than half of the rhesus macaques at Oregon National Primate Research Center were experiencing hair loss, and that 15 monkeys had been injured in a fight that left six of the animals dead, OHSU said.

In a statement issued on Thursday, OHSU said Oregon National Primate Research Center officials "believe in an ethical mandate to ensure that all of our animals are treated respectfully and humanely."

Another monkey had died during surgery due to a mistake with an anesthesia machine, and inspectors found muddy bedding and cracked walls in some monkey housing areas, OHSU said.

OHSU spokesman Todd Murphy said the hospital, which was not fined or otherwise formally penalized, has already moved to correct problems highlighted in the warning.

Over the past six months they have taken steps to prevent future monkey fights such as changing their food, they have invested in improved medical equipment, and they have made upgrades to primate housing, OHSU said in an earlier statement.

OHSU also emphasized in multiple documents about the primate research center that the lab was recently re-accredited by federal regulators.

The primate center, located about 15 miles from OHSU's main hospital campus, houses 5,000 animals, which are used to research diseases, neurology and reproductive biology.

Stop Animal Exploitation Now, an anti-animal-laboratories group that publicized the federal warning, has asked Department of Agriculture regulators to fine OHSU for its treatment of primates.

(Reporting by Courtney Sherwood in Portland, Oregon; Editing Eric M. Johnson and Sandra Maler)