President George W. Bush and Russian President Vladimir Putin agreed Saturday to work together to combat nuclear terrorism, saying the greatest threat facing the world today is the threat of attack by nuclear, chemical or biological weapons. The accord was to be announced at a joint news conference by the two leaders before the opening of a summit of world leaders. The anti-terrorism initiative builds on an existing "Proliferation Security Initiative," a U.S.-led group of dozens of nations that have pledged to help seize illicit weapons as they are transported around the world. Bush and Putin greeted each other warmly at the open of their talks. Putin said it was time "to synchronize our watches." Putin told Bush, "We count on your support during this international event." The new program, known as the Global Initiative to Combat Nuclear Terrorism, envisions other countries joining forces with the United States and Russia to combat the threat, a U.S. official familiar with the program was quoted as saying by the Associated Press. The program calls on states to improve accounting, control and physical protection of nuclear material and radioactive substances as well as the security of nuclear facilities, the official said. The U.S. and Russian presidents will be joined by the leaders of Britain, Canada, France, Germany, Italy and Japan at the annual G-8 summit, which begins with dinner Saturday night and continues through Monday. This is Russia's first year as host.