VATICAN CITY — Pope Benedict XVI said divisions and conflicts “cast dark shadows” on the future of humanity, warning of the danger coming from more countries possessing nuclear weapons.

He urged authorities to step up negotiations for a “progressive and mutually agreed dismantling of existing nuclear weapons.”

“Humanity today is unfortunately experiencing great division and sharp conflicts which cast dark shadows on its future,” Benedict said in his annual message to world leaders for the Roman Catholic Church’s World Day of Peace on January 1.

He said, “The danger of an increase in the number of countries possessing nuclear weapons causes well-founded apprehension in every responsible person.” He did not name specific countries, although he lamented civil wars in Africa and said the Middle East “is still a theater of conflict and violence” affecting neighboring nations and regions.

Cardinal Renato Martino, who presented the pope’s message at a news conference, was asked if the pope was referring to concerns raised by Iran’s nuclear program. He said “you could imagine” the message referred to current issues but added that “the Holy See never makes specific reference but brings up the problem.”

Meanwhile, President Bush called on Iran to explain why it had a secretive nuclear weapons program and warned that any such efforts must not be allowed to flourish “for the sake of world peace.”

“We believe Iran had a secret military weapons program, and Iran must explain to the world why they had a program.”

Mr. Bush’s comments came after President Ahmadinejad of Iran said it was “a step forward” that American intelligence agencies had concluded that Tehran stopped developing its nuclear weapons program four years ago. Mr. Ahmadinejad said an “entirely different” situation between America and Iran could be created if more steps like the intelligence report followed. “We consider this measure by the U.S. government a positive step. It is a step forward,” Mr. Ahmadinejad said. “If one or two other steps are taken, the issues we have in front of us will be entirely different and will lose their complexity, and the way will be open for the resolution of basic issues in the region and in dealings between the two sides,” he said.

The White House press secretary, Dana Perino, dismissed Mr. Ahmadinejad’s comments as “fanciful thinking.”