A senior US State Department official called for restraint after Palestinian Hamas spiritual leader Sheik Ahmed Yassin was killed in Israeli missile attacks early on Monday.  

"The United States urges all sides to remain calm and exercise restraint," said the official late Sunday on condition of anonymity.

He also said that Washington was following developments in the Middle East and were in touch with Israeli and Palestinian authorities.

Israel Apache helicopters fired three missiles at Yassin and his two body guards as they left a mosque and killed them instantly.

Senior Hamas leader Abdul Aziz al-Rantissi has declared "open" war with Israel after Yassin was killed.

(Xinhua News Agency March 22, 2004)