Israeli Prime Minister Ariel Sharon has ordered his security forces to go after the leaders of Islamic Jihad after a suicide bombing by Palestinian militant group killed four Israelis Tuesday.

Mr. Sharon told reporters he ordered troops to step up operations against the group and "hit the leadership."

Hours after the deadly attack in Netanya, Israeli forces sealed off the West Bank and Gaza Strip. Isreali troops also re-occupied the West Bank city of Tulkarem, where they arrested five suspected Islamic Jihad militants and killed a Palestinian police officer.

Meanwhile, Israeli soldiers are stopping non-residents from entering Jewish settlements in the Gaza Strip. Mr. Sharon ordered the measure to prevent mass demonstrations by those opposed to the planned Israeli withdrawal from Gaza.