Crawford, Texas ? President Bush and Japanese Prime Minister Junichiro Koizumi warned North Korea Friday that any escalation of its nuclear weapons program would prompt “tougher measures” by the United States and Japan.

They avoided defining what measures they had in mind if Pyongyang moves ahead anyway, and offered few clues about what would trigger U.S. and Japanese action. In some cases where the leaders did give hints, North Korea has already apparently defied the two governments.

“We will not at all tolerate the possession, the development or the transfer of nuclear weapons by North Korea,” Koizumi said. The CIA believes North Korea already has one or two nuclear weapons, and North Korea said during talks in Beijing last month that it might test, sell or use its claimed arsenal.

Bush demanded “the complete, verifiable and irreversible elimination of North Korea’s nuclear weapons program.”

Any number of North Korean actions could represent escalation of the nuclear program, including proof that North Korea has reprocessed spent nuclear reactor fuel into weapons-grade plutonium, a senior Bush administration official said.

North Korean has said it reprocessed some 8,000 rods, but U.S. intelligence agencies say they have no recent evidence of it.

Launches of North Korean rockets toward Taiwan also could constitute escalation, the official said, speaking on condition of anonymity. North Korea in February fired a missile into the Sea of Japan.