The United States has changed its demand for on-site inspection of suspected missile sites and manufacturing plants as part of a proposed treaty banning U.S. and Soviet medium-range missiles, the State Department said yesterday.

Department spokeswoman Phyllis Oakley said the changes in the U.S. proposals, made yesterday at arms talks in Geneva, are possible because of the Soviet agreement that all medium-range weapons worldwide should be eliminated.

Previous verification proposals had been based on the assumption that each side would retain 100 warheads and their missiles outside Europe. Oakley said a missile ban is easier to verify than provisions allowing a limited number of missiles, which would have to be monitored.

But she also noted, "The United States has not softened its verification inspection measures." Instead, she said the U.S. position on verification has been modified in two ways:The U.S. demand for surprise inspections of suspected missile sites has been changed to become less stringent. The United States has dropped a proposal for "perimeter portal monitoring," arms-control jargon for devices such as cameras or scales that check manufacturing plant gates to ensure that no banned missiles are being shipped.

She said the U.S. insistence on such monitoring remains part of the treaty talks on long-range weapons.

The new U.S. verification proposals in Geneva are based on elimination of all medium-range missiles within three years, according to Oakley. She said they are also based on a ban on modernization, production and test flights of such weapons in the interim.

She said the United States proposed destroying all missiles with ranges of 300 to 600 miles within a year of an accord, which is expected to be completed this year.

Missiles with ranges of 600 to 3,000 miles should be scrapped within three years, she said.