JERUSALEM, Aug. 27 (JTA) — Israeli officials stepped up security in Tel Aviv and Jerusalem after a bomb filled with nails exploded on a busy Tel Aviv street during Thursday’s morning rush hour. At least 21 people were injured in the attack, including two young children. The bombing occurred amid reports earlier in the week that progress had been achieved in Israeli-Palestinian negotiations. In the wake of the attack, police reinforcements were dispatched to beaches, shopping and entertainment centers, and pedestrian malls in the country’s two largest cities. Security checks at Ben-Gurion Airport were intensified. Israeli leaders of all political stripes were soon demanding that the Palestinian Authority condemn the attack. Palestinian leader Yasser Arafat did so — but only in a private conversation with Knesset Member Saleh Tareef of the Labor Party. He said he condemns those who attack Israeli citizens. In the past, Palestinian officials, including Arafat, have made a distinction between Israelis and settlers. The Palestinian leader also said the explosion may have been the work of common criminals, not terrorists. Arafat told Tareef that he did not share the optimistic reports published earlier in the week that Israel and the Palestinians were nearing agreement for a further Israeli redeployment in the West Bank. He said there is still a long way to go “because of Netanyahu’s positions.” Meanwhile, Prime Minister Benjamin Netanyahu warned Thursday that unless the Palestinian Authority acts against terrorism “in both deeds and words,” there will be no agreement. Netanyahu made the comment to reporters while visiting the hospital bedside of one of the attack’s victims, Michal Matityahu. The prime minister pointed out that although Arafat has denounced attacks on Israeli citizens, the Palestinian leader has not yet condemned the murders earlier this month of two Israelis in the West Bank because the victims were settlers. The bomb that exploded Thursday had been placed in a garbage can at one of Tel Aviv’s busiest intersections, the corner of Allenby Street and Rothschild Boulevard. It was the first attack in Tel Aviv since March 21, 1997, when a Hamas suicide bomber detonated an explosion at the Cafe Apropo, killing three Israelis and wounding 47 others. “Suddenly a very strong explosion occurred,” recalled David Cohen, who was sitting inside a coffee house when the bomb exploded about five feet away. Cohen was only slightly injured. Only one of the victims, Matityahu, required hospitalization at Ichilov Hospital in Tel Aviv. All the others suffered light injuries. The explosion, which shattered windows in some 80 apartments and 10 stores, caused damage estimated at $400,000. As of Thursday evening, no one had claimed responsibility for the attack. Hamas leader Sheik Ahmed Yassin said the explosion was probably the work of Jewish extremists seeking to derail the peace process. One day before the explosion, Yassin called for attacks on Israel to retaliate for last week’s U.S. missile strikes in Sudan and Afghanistan. A few hours after the explosion in Tel Aviv, thousands of Israelis gathered in the West Bank city of Hebron to mark the end of the shiva period for Rabbi Shlomo Ra’anan, who was killed in his home last week by a suspected Palestinian intruder. Angry mourners attacked Palestinian reporters who were on the scene to cover the ceremony. Clashes involving Jewish settlers and Palestinians in Hebron have increased after Ra’anan’s murder. Tensions there escalated after the Israeli army responded to the killing by imposing a round-the-clock curfew on the estimated 30,000 Palestinians living in the Israeli-controlled part of the heavily Arab city. Meanwhile, Israeli officials disclosed Thursday that security forces recently uncovered an underground cell of 20 Palestinians operating in the Hebron region. The group reportedly confessed to carrying out several shootings on Jewish targets in the West Bank. The cell is said to be part of the Abu-Mussa group, which had split from the mainstream Fatah Party and is headquartered in Damascus.