The armed wing of the Palestinian group, Hamas, has called on members to avenge the death of two of its leaders killed in an Israeli army raid. Two other Palestinians and an Israeli soldier were killed after Israeli troops surrounded a building in a refugee camp in Nablus, where suspected Hamas activists were staying. Senior Hamas leaders said the group was still committed to its ceasefire, but warned that "violations" would not go unanswered. This is one of the most serious incidents since Palestinian groups declared a unilateral ceasefire five weeks ago. Hamas' Izzedine al-Qassam Brigades identified their dead as Fayez Assader, 26, a leader in Nablus, and Khamis Abu Salem, 22. In a statement, the group said: "We call on all cells to answer this crime and teach the enemy the right lesson. "The crimes of the Zionist enemy, perpetrated against our people, and its daily violations will not go without the enemy paying a commensurate price." **Bystanders Killed*** Israeli military sources said when they approached the building early on Friday in the Askar refugee camp in Nablus, a known Hamas stronghold, they were shot at from its third floor. Israeli soldiers responded by reportedly firing an anti-tank missile at the building, setting off several explosions. Hundreds of people gathered to look at the wreckage of the building destroyed before the Israeli troops withdrew. A 20-year-old Palestinian was killed when Israeli troops opened fire on people throwing stones at them, and another bystander died from the effects of tear gas sprayed by soldiers. **PHOTO CAPTION*** A masked Palestinian from Jenin Martyrs Brigade, Thursday Aug. 7, 2003. (AP Photo/Adel Hana)