Some 80 percent of Gaza's factories have temporarily shut down in the past month, after Israel closed border crossings in response to the Hamas takeover of the coastal strip, human rights groups and Palestinian industrialists said Sunday. The factory closures make Gaza's 1.5 million residents increasingly dependent on humanitarian aid, said Michael Bailey of Oxfam International, an aid and human rights group. Before the latest closures, some 85 percent of Gazans received some form of aid, he said. If the crossings between Gaza and Israel aren't opened soon, "the slide into all-out dependency will be swift and inevitable," he said. Israel said it keeps Gaza's main cargo crossing closed for security reasons and because it cannot coordinate truck shipments with Gaza's Hamas rulers. Israel refuses to talk to Hamas, a terrorist organization that has killed hundreds of Israelis in suicide bombings and other attacks in recent years. Since the fall of Gaza to Hamas, Israel has only permitted food and medicine to enter Gaza, said Bassim Khoury, head of the Palestinian Federation of Industries. Palestinian businesses have been unable to import raw materials or export goods. Khoury said 80 percent of Gaza's factories have temporarily shut down in the past month, and those still operating run at only 60 percent capacity. Khoury noted that 120,000 Gazans work in manufacturing, and that many have been laid off. One-third of Gazans were already unemployed before the latest crisis, and the layoffs will further weaken poverty, he said. Mohammed al-Talbani, who owns the Al Auda cookie factory in Gaza, said he has laid off 270 of his 370 workers. "It is a mistake to think that choking Gaza will work against Hamas," he said in a statement. "Quite the contrary - the economic stranglehold is driving people to extremism. In Gaza, people receive food assistance from Hamas, and they are blaming Israel for the closure." Oxfam and the Israeli human rights group Gisha urged Israel to reopen the Karni crossing from Israel for cargo, which has served as Gaza's lifeline. They also asked that the Quartet of Mideast mediators - the US, the UN, Europe and Russia - deal with Gaza's economic crisis at their meeting in Portugal next week. However, Shlomo Dror, a spokesman for Israel's coordinator of policy in the Palestinian territories, said Israel is doing its best under difficult circumstances to avert a humanitarian crisis in Gaza. He noted that Hamas militants have fired at Gaza crossings, and that this makes it difficult to increase truck shipments, Dror said Israel recently allowed 3.5 million flower seedlings to be shipped to Gaza's farmers.