Feb 24 04:41 AM US/Eastern LONDON (AP) - The United States, European and Arab nations are set to deliver a stern warning to Syrian President Bashar Assad (bah-SHAR' AH'-sahd). Diplomats say he must agree to an immediate cease-fire and allow humanitarian aid into areas hardest hit by his regime's crackdown on opponents, or face as-yet unspecified punishments. On the eve of a major international conference on Syria in Tunisia, U.S., European and Arab officials worked out details of the demands. Also, the former United Nations chief, Kofi Annan, was named to be an envoy to deal with the crisis. Russia and China reiterated their opposition to an international resolution. Diplomats say the so-called Friends of Syria group meeting Friday in Tunis will demand Assad's compliance. They say failure on his part will result in tougher sanctions.