Israel says it has arrested some 55 Palestinians wanted in connection with terrorist activities in operations over the past 24 hours. Among those arrested is a long-sought commander of the Al-Aqsa Martyrs Brigades.

Israeli authorities say the man, Majid al-Masri, was arrested near the West Bank city of Nablus. His detention, and that of the others rounded up, followed a renewed pledge by Prime Minister Ariel Sharon to hunt down those responsible for attacks on Israelis.

The Al-Aqsa Martyrs Brigades, an offshoot of Yasser Arafat's Fatah faction, claimed responsibility for the attack on the northern Israeli town of Beit Shean on Thursday night, in which six Israelis were killed and more than 20 were wounded.

Israel has been looking for Mr. Al-Masri for more than two years. They say he recruited terrorists and planned numerous attacks. He also was a spokesman for the Al-Aqsa Brigades and was a former Palestinian policeman.

Also on Saturday, Palestinian medical workers said a 16-year-old youth was killed by fire from Israeli troops in the Gaza City area.

Israel is still reeling from two attacks in Kenya on Thursday. In one incident, three suicide bombers killed themselves and 12 others, including three Israelis, when they blew up an Israeli tourist hotel near Mombasa. The second attack was a near miss, which occurred when two missiles were fired at an Israeli airliner as it took off from Mombasa airport.

Meanwhile, U.N. Secretary-General Kofi Annan has sent a letter to Israeli Prime Minister Ariel Sharon demanding that Israeli troops be punished for killing a U.N. worker during a clash with gunmen last week.

Press reports in Israel say Mr. Annan told the Israeli leader he expects Israel to carry out a rigorous investigation of the shooting.

A Foreign Ministry spokesman was quoted as saying on Saturday that results of the army's investigation into the killing of British aid worker Iain Hook could be expected within the next few days.

Israel has said Mr. Hook was shot after Israeli troops returned fire at gunmen shooting at them from inside the U.N. compound in Jenin.

U.N. officials say there were no gunmen inside the building.