A day after an Islamic Jihad suicide bomber blew himself up in South Tel Aviv Monday, killing nine people and wounding close to 70, IDF troops set up roadblocks outside Nablus, Jenin and Tulkarm and sealed off Samaria from the rest of the West Bank. The decision to seal off Samaria Tuesday followed a security assessment meeting held by Interim Prime Minister Ehud Olmert, Defense Minister Shaul Mofaz, IDF Chief of General Staff Lt.-Gen. Dan Halutz and Shin Bet (Israel Security Agency) chief Yuval Diskin. Olmert also decided to crack down on Palestinians illegally in Israel and to speed up construction of the Jerusalem Envelope barrier. Security sources believe Monday's bomber infiltrated Israel through gaps in that barrier. The Shin Bet recorded more than 70 general terror alerts during the Pessah holiday, in addition to 17 concrete terror threats, most of them originating in Nablus, Jenin and Tulkarm. The IDF stepped up raids in Palestinians cities and arrested more than 40 terror suspects affiliated with Islamic Jihad, Fatah and the Popular Front for the Liberation of Palestine overnight Tuesday. Security forces clashed with hundreds of Palestinian rioters in Nablus on Wednesday, firing tear gas and rubber bullets to push back the crowds. Two Palestinians were wounded in the clashes, witnesses said. Earlier Wednesday in the city, soldiers employing some 30 jeeps searched for wanted Palestinians and briefly detained four female relatives of suspects, Palestinian Authority security officials said. A 19-year-old Palestinian was caught with five pipe bombs at a checkpoint west of Nablus. Sappers safely detonated the bombs and the man was transferred to the Shin Bet for further questioning. Meanwhile, IDF artillery fired more than 420 shells at Kassam launch sites in the northern Gaza Strip after two Kassam rockets were fired over the Green Line. One rocket landed in an open field near Kibbutz Karmiya. Palestinians also fired an anti-tank rocket at troops patrolling the Gaza security fence. No one was wounded in the attacks.