Hackers hit Tel Aviv Stock Exchange, El Al TEL AVIV, Israel, Jan. 16 (UPI) -- A cyberattack knocked the Tel Aviv Stock Exchange and El Al Web sites off-line Monday, a day after a hacker network threatened to breach both sites. The network, called "Nightmare Group," caused various problems for both sites, Haaretz reported. A later check indicated the stock exchange site was still disrupted but the airline's was back in service. A representative of the exchange said the trading system and the Web site were separate, stressing that trading was not damaged by the attack. El Al officials said the airline was "taking safety measures in the operations of the company's Web site" and disruptions were expected. The breach follows a series of cyberattacks during the past two weeks and a day after Hamas called for harsher hacking attempts against Israeli Web sites, Haaretz said. "The penetration into Israeli Web sites opens a new front for electronic resistance and war against the Israeli occupation," Hamas representative Sami Abu Zuhri said during a news conference in Gaza Sunday. The hacker 0xOmar, who released tens of thousands of Israeli credit card numbers last month, sent an e-mail to The Jerusalem Post before the attack saying he was joined by the Nightmare hacking team. Israeli hackers recently said they would launch their own attacks against Arab hackers, the Post said. A Jewish pro-Israel hacker named Hannibal published information enabling Internet users to hack the accounts of 20,000 Arab Facebook users.