JERUSALEM, Jan. 3 (JTA) — U.S. officials welcomed Israel´s move to ease restrictions on the Palestinians just as special envoy Anthony Zinni arrived in the region. Acceding to an American request, Israeli troops began withdrawing from Palestinian-controlled areas in Jenin and parts of Ramallah. Troops also withdrew from Nablus, though Israel left open the possibility that patrols would continue in the city if necessary. A blockade around the cities Hebron and Kalkilya was also lifted. Further measures were planned, including pulling back tanks from areas of friction. The United States called the moves a "positive step." Zinni returned to the region Thursday in another attempt to forge a cease-fire between the Israelis and Palestinians. His last mission was derailed by a wave of suicide bomb attacks against Israel, which led to international pressure on the Palestinians to crack down on terrorism. But his new efforts will not be easy. Despite a decrease in violence during the past two weeks, Israel maintains that Palestinian Authority President Yasser Arafat has not done enough to crack down on terror. Prime Minister Ariel Sharon has insisted that seven days of absolute quiet be observed before any political talks can begin. The Palestinians, meanwhile, say they have been clamping down on terror and accuse Sharon of seeking a pretense to avoid resuming political talks. At the same time, there are deep differences within Sharon´s government over what policy to adopt toward the Palestinians. Upon his arrival Thursday, Zinni met with Israeli security officials in Tel Aviv. He was scheduled to breakfast Friday with Sharon at his Negev ranch. They were to be joined by Foreign Minister Shimon Peres and Defense Minister Benjamin Ben-Eliezer. Later Friday, Zinni was to hold talks with Palestinian Authority leader Yasser Arafat in Ramallah. In light of the situation, analysts in Israel speculated that the current shuttle mission will not aim at getting Israeli and Palestinian commitment to begin implementing any cease-fire accords, but to lay the groundwork for them to do so on his next mission. Despite Israel´s moves to ease restrictions in the territories, security forces continued operations against suspected terrorists Thursday. Four Islamic Jihad members suspected of involvement in terrorist activity were detained in a raid in the Palestinian-controlled part of Hebron. A Hamas activist was detained near Tulkarm. Arafat, meanwhile, dismissed the Israeli moves to ease restrictions. In remarks Thursday, he called for observers to go to the areas Israel mentioned "and see if the closure has really been lifted." At the same time, the Palestinian Authority reportedly warned Islamic groups against carrying out suicide attacks during the envoy´s visit. In another development, the Shin Bet security service revealed that Hamas members in Saudi Arabia had been sending money, instructions and information to counterparts in the Gaza Strip to produce rockets. A Hamas member who served as a liaison between the sides was detained at the Rafah Crossing.