The United States on Friday rejected an offer by Libyan leader Moammar Gadhafi to allow international inspection of a plant that allegedly produces chemical weapons.

The Libyan offer was made last week publicly by Gadhafi and in a message conveyed to the United States by Italian Foreign Minister Giulio Andreotti.The United States said it has evidence that the factory that the Libyans call Pharma-150 is in fact on the verge of producing chemical weapons. Libya claims the newly built facility near the town of Rabta, 50 miles southwest of Tripoli, is in fact a pharmaceutical plant.

"A one-time inspection could not be conclusive in this regard," State Department spokeswoman Phyllis Oakley said in rejecting Gadhafi's offer.

"A CW plant could easily be modified to appear as a legitimate industrial chemical plant such as a pharmaceutical or fertilizer facility," she said.

"All traces of chemical weapons production could be erased from a plant on extremely short notice," she added.

President Reagan said last week that the United States had discussed with other NATO members possible military action against the plant, and had urged the allies to withhold technical expertise from the Libyans that could be used for chemical weapons production.

Companies from Italy, Japan and West Germany are believed to have helped Libya in building the plant or the adjacent industrial complex. Japan told the United States its nationals believed they were involved in the construction of a fertilizer plant.

CIA Director William Webster said recently that the Libyan plant is on the verge of production, and Oakley said Friday it was not yet operational.

The United States also makes chemical weapons as a deterrent against such an attack from the Soviet Union. But U.S. counterterrorism officials have expressed concern that Gadhafi might give chemical arms to one of several terrorist groups which make their headquarters in Tripoli.

The Libyan plant will be discussed next week at an international conference in Paris on plans to stem the proliferation of chemical weapons around the world. The U.S. delegation will be led by Secretary of State George P. Shultz.

Foreign ministers at the Paris meeting also will discuss claims by South Korea that North Korea has chemical weapons capability.

Oakley said U.S. information shows the communist North does have limited chemical weapons capability based on equipment supplied by the Soviet Union.

But she said the United States could not confirm or deny the existence of production or storage facilities in the North because its most recent information was from 1986.