Hamas's armed wing, Izzadin Kassam, has vowed to abide by the cease-fire agreement with Israel, Hamas officials said Wednesday. The armed group, which reports directly to Damascus-based Hamas chief Khaled Mashaal, had instructed all its members to refrain from firing rockets at Israel as of Thursday morning, the officials said. Again according to the officials, the decision to reappoint Said Siam as interior minister in the Hamas government was taken by Mashaal and the Hamas leadership in Syria. One of Siam's main tasks was to supervise the work of Izzadin Kassam and make sure that its members remained loyal to the Damascus-based Hamas leaders, the officials explained. Siam is one of the few Hamas leaders in the Gaza Strip who commands the full respect of Izzadin Kassam and its commander, Ahmed Ja'bari. "Ja'bari does not receive his instructions from Prime Minister Ismail Haniyeh or other political figures in Hamas," one official said. "He is linked to Mashaal through Siam and [former Hamas foreign minister] Mahmoud Zahar." Another Hamas official said Izzadin Kassam had also been entrusted with enforcing the cease-fire agreement. Members of the group, along with other Hamas security forces, will be deployed in sensitive areas to make sure that other groups don't fire rockets at Israel. Some Hamas officials expressed concern on Wednesday that Islamic Jihad, Fatah and armed groups in the Gaza Strip would try to embarrass Hamas by launching attacks on Israel. The leaders of all the armed groups in the Gaza Strip had been warned by Hamas against violating the cease-fire, the officials said. Islamic Jihad representatives voiced reservations about the cease-fire agreement on Wednesday, but said they would nevertheless honor it. Nafez Azzam, a top Islamic Jihad leader in the Gaza Strip, said his organization opposed the agreement because it did not include the West Bank in the first phase. He said his organization also had reservations about the agreement because it was not clear about the Rafah border crossing. Haniyeh praised the agreement as an achievement for Hamas, saying it would end the Israeli "aggression" against the Palestinians. "This tahadiyeh [period of calm] will provide security and alleviate the suffering of our people," he said. "It will also bring quiet to the Israelis if they abide by it." Haniyeh warned Israel against violating the agreement, saying Hamas reserved the right to respond to any breach. Asked if and when the cease-fire agreement would be implemented in the West Bank, Haniyeh said Hamas had received assurances from Egypt that this would happen at a later stage. In response to reports that Palestinian Authority President Mahmoud Abbas was planning to visit Gaza soon, Haniyeh said, "The Gaza Strip welcomes anyone who wants to come here," he said. "If the president wants to visit us, he is most welcome."