Philippine senators, backing down from a tough anti-American stand, Monday agreed in principle to amend a bill banning nuclear weapons in the country and allow entry to nuclear-powered vessels, a senator said.

Sen. Wigberto Tanada said the upper house made the decision at a closed-door meeting to tone down the anti-nuclear bill that U.S. officials fear could lead to the dismantling of major American installations in the Philippines.The wide-ranging bill in its present form bars the "development, manufacture, acquisition, testing, use or storage" of nuclear arms and components. It also prohibits the entry of "nuclear-armed, nuclear-powered or nuclear-capable military overland transporters, ships or seaborne vessels or submarines, or aircraft or airborne vehicles."

"I'm a little bit sad because the term `nuclear-powered' as well as the term `nuclear-capable' were deleted or excluded from the ban," said Tanada, a member of President Corazon Aquino's ruling coalition who sponsored the anti-nuclear bill.

However, Tanada told reporters the 21/2 hour caucus agreed that "before these nuclear-powered or armed vessels or aircraft are allowed to enter our territory, they would be subject to verification and inspection."

Tanada stressed under the proposed amendments the carrying by ships of nuclear weapons will be banned. However, he said nuclear-powered ships and vessels capable of carrying nuclear weapons will be allowed entry provided there is prior inspection by Philippine authorities.

Those conditions could still cause severe difficulties for the United States, which maintains six military bases in the Philippines. Washington has a standing policy of refusing to confirm or deny the presence of nuclear arms in its facilities or whether its ships or aircraft carry nuclear weapons.

U.S. military officials said it would be "extremely difficult" to operate Clark Air Base and Subic Bay Naval Base if the anti-nuclear bill becomes law. Clark and Subic house more than 17,000 servicemen plus dependents and are the largest U.S. military facilities outside continental United States.