US President George W. Bush and Russian President Vladimir Putin agreed Saturday on a global initiative to combat nuclear terrorism, saying the greatest threat facing the world 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 US-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."