JERUSALEM — Israel faced an escalating cyberwar on Monday as unknown attackers disrupted access to the symbolically strategic Web sites of the Tel Aviv Stock Exchange and El Al, the national airline.

A hacker identifying himself as oxOmar, already notorious for posting the details of more than 20,000 Israeli credit cards, sent an overnight warning to Israel’s Ynet news outlet that a group of pro-Palestinian cyberattackers called Nightmare planned to bring down the sites in the morning.

The attackers did not break into the sites’ operating systems, but used a far simpler tactic: creating an overload of access attempts. Neither the Israeli economy nor flights in and out of the country were endangered, and the sites appeared to be recovering within hours, but the assault left many Israelis feeling vulnerable.

Yoni Shemesh, who is responsible for the Tel Aviv Stock Exchange Web site, said that his team had already begun preparing for an attack a few days ago and went on high alert after the Ynet report.