GAZA CITY, Gaza Strip - Israel killed four Hamas militants in a missile strike Saturday and moved artillery to the Gaza border, launching what it vowed would be a "crushing" response to a Hamas rocket barrage on Israeli towns.

Israel also sealed the West Bank and Gaza, barring all Palestinians from its territory, within hours of the Hamas attack - the group's first major violence since Israel withdrew from the Mediterranean coastal strip two weeks ago.

Prime Minister Ariel Sharon called his Security Cabinet for a meeting later Saturday to approve the military action, expected to last several days. A large-scale operation appeared unlikely but the timing of the meeting suggested a sense of urgency.

The Cabinet session comes as Sharon faces a major leadership challenge in his Likud Party over the Gaza withdrawal, completed two weeks ago. Sharon's challenger, former Prime Minister Benjamin Netanyahu, has warned the pullout will endanger Israel, and the barrage of 39 rockets, with five Israelis wounded, could boost his agenda.

The escalation followed an explosion Friday at a Hamas rally at a crowded Palestinian refugee camp that killed at least 15 Palestinians. Witnesses said the blast went off near a pickup truck carrying masked militants and homemade rockets.

Hamas blamed Israel and said it fired rockets on Israeli border towns in retaliation. But the Palestinian Authority said the blast was an accident resulting from militants mishandling explosives. It renewed demands that armed groups stop flaunting their weapons.