WASHINGTON - The Occupational Safety and Health Administration, in its largest enforcement action ever, said today it will seek more than $1.3 million in fines against Union Carbide Corp. for 221 alleged safety and health violations at its Institute, W.Va., chemical plant.

OSHA officials said the citations were issued this morning at the plant, where a chemical leak last August hospitalized six workers and sent 129 area residents to emergency rooms.

The company has 15 days to contest the fines, which totaled $1,377,300.

If it does, the case would be adjudicated first before an administrative law judge and then before the three-member Occupational Safety and Health Review Commission.

Union Carbide last month agreed to pay OSHA fines totaling $4,400 to settle alleged violations in connection with the Aug. 11 accident. That fine had been reduced from an initial $30,000 after the company promised to improve monitoring systems at the plant.

OSHA said the alleged violations stem from a six-month investigation by a 15-member team and that some involve record-keeping problems over several years.

OSHA officials in Washington said the new fines were levied after inspectors completed the first phase of a wall-to-wall inspection of the plant launched after the accident.

Labor Secretary William E. Brock called the proposed fines "a necessary and appropriate response to what we believe to be a willful disregard for health and safety."

While Brock acknowledged that the Reagan administration has sought what he called a "balanced approach" with employers, he sharply criticized Union Carbide's performance.

"We will impose the full penalties of the law on those who blatantly or repeatedly violate safeguards necessary to protect American workers," Brock said.

Pat Tyson, assistant OSHA director, said the company is under no time constraint to correct the problems. He said a timetable will be negotiated with the company.

Tyson said area residents are not in great danger even though the problems were in systems that would control a poisonous gas leak. "We'd have to have a series of major problems before we get that far," he said.

Among the eight violations listed as willful by OSHA was failing to provide respirators to employees exposed to phosgene, an extremely deadly gas. The agency said the workers "were required to sniff to determine the presence of this deadly gas."

OSHA said Union Carbide had repeatedly failed to record instances of injuries requiring medical treatment or resulting in restricted activity or lost work days and also failing to provide what it called critical supplemental medical information on asbestosis cases.

OSHA inspectors went to the plant a month after the leak of aldicarb oxime and methylene chloride. The agency announced plans to inspect 18 units; today's fines involve five.

In all, OSHA's report accused Union Carbide of 221 violations of 55 safety and health laws. Among the alleged violations:

Inability to track leaks from chemical producing systems.

Inadequate backup safety systems.

Potential for fire and explosion.

Potential for chemical overexposure of workers.

Hord said today's news conferences in Washington and Charleston were called so officials could meet statute-of-limitation requirements.

"Once a hazard is uncovered, you have to announce any violations within six months. We had until April 7 to announce anything found."

Union Carbide spokesman Thad Epps said he had expected OSHA to announce some problems.

Plant spokeswoman Cathy Jones said plant manager Rudy Shomo received the OSHA report this morning. She said Shomo and other plant officials were reviewing the information and would comment later.

Just before the August leak, OSHA inspectors reviewed $5 million in safety equipment installed at the Institute plant's methyl isocyanate unit. MIC is the same gas that killed more than 2,000 people when it leaked from Union Carbide's Bhopal , India, plant on Dec. 3 , 1984.

Union Carbide's Institute plant produces intermediate" chemicals that are then used at other plants to produce pesticides and other agricultural chemicals. The company's plant in Texas City is largely engaged in the production of solvent chemicals and handles substances that are generally less toxic than those at Institute, said Jim Hockersmith, spokesman for the Texas City plant.

As a result of concerns raised by the Bhopal and Institute accidents, OSHA in January began a series of special safety inspections of chemical plants at selected locations around the country.

The inspections include examinations of operations involving very hazardous chemicals. One chemical is being considered at each of 50 to 60 chemical plants in the Houston area and nearby areas.