Jerusalem ? Hamas founder Ahmed Yassin brushed off warnings by a top Israeli official that he is “marked for death” and, in a defiant appearance Friday at a Gaza City mosque, said his Islamic militant group would continue to attack Israelis.

The exchange of threats after a deadly Hamas bombing this week further spoiled chances for a cease-fire, the starting point for a U.S.-backed peace plan.

Israeli officials said targeted killings of senior Hamas members would likely resume after Wednesday’s bombing killed four Israeli border guards at the Erez crossing between Gaza and Israel. Pinpointed Israeli strikes against Palestinian militants have triggered revenge bombings in the past.

Deputy Defense Minister Zeev Boim initially told Israel Army radio that Yassin would be targeted — the first time an Israeli official was so specific.

“Sheik Yassin is marked for death, and he should hide himself deep underground where he won’t know the difference between day and night. And we will find him in the tunnels, and we will eliminate him,” Boim told Israel Army Radio.

But Boim later said high-level Defense Ministry discussions on a response to the Erez bombing did not include a specific decision to kill Yassin.

Security officials said killing Yassin would require approval from Prime Minister Ariel Sharon. “We never comment on specific cases,” said Sharon adviser Raanan Gissin.

In the summer, Israel unleashed several airstrikes against leading Hamas figures, killing one but missing three others — including Yassin, who escaped the bombing of a Gaza City building in September with minor injuries. The 69-year-old Hamas founder is a quadriplegic from a childhood sporting accident.

In more than three years of fighting, Israel has killed more than 140 suspects but refrained from going after top Palestinian leaders. It has threatened to “remove” Yasser Arafat, suggesting it would either exile or kill him but has refrained from doing either, in part because of U.S. opposition.

An Israeli strike against Yassin would likely provoke bloody Hamas attacks and boost support for the group, Arafat’s main political rival, said Palestinian legislator Hanan Ashrawi.

In Washington, State Department spokesman Richard Boucher reiterated U.S. policy against targeted assassinations and criticized Hamas for violent acts that make peacemaking more elusive.

Palestinian supporters of Hamas Spiritual leader Sheik Ahmed Yassin hold a poster of Yassin as they crowd the doorway Friday in front of his home in Gaza City. Israel has warned Yassin that he tops the list of militants to be hunted and put to death. The threat comes after a Palestinian suicide attacker blew herself up this week at a crossing point between Israel and the Gaza Strip, killing four Israelis.